catch errors when encoding entity map

Summary:
We've had several reports of people's content failing to save in the Performance tool. The only error message we have is this (with no callstack, unfortunately):
```
Unknown DraftEntity key: undefined.
```
I poked around in the DraftJS code, and my guess is that this is due to some character having an entity key associated with it that for whatever reason does not appear in the entity map. This diff "fixes" the issue by not throwing an error in this situation.

This is definitely not as good as figuring out the root cause of the entity mismatch, but it seems reasonable for `convertFromDraftStateToRaw` to work with slightly malformed input.

Not sure if it's ok to use `FBLogger` in this code or not, but I saw another usage of it in `draft-js-contrib` here:
diffusion/WWW/browse/master/html/shared/draft-js-contrib/matchers/getImplicitDateTimeMatches.js;1000956292$34

I could also just ignore the error, but the linter didn't like that ¯\_(ツ)_/¯

Reviewed By: niveditc

Differential Revision: D16362778

fbshipit-source-id: 6e2a6041d1a02b412d94655c9b6224fee868748d

Author: Frank Thompson

src/model/encoding/__tests__/__snapshots__/convertFromDraftStateToRaw-test.js.snap

@@ -1,5 +1,28 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`must be able to convert from draft state entities not in map 1`] = `
+Object {
+  "blocks": Array [
+    Object {
+      "data": Object {},
+      "depth": 0,
+      "entityRanges": Array [
+        Object {
+          "key": 0,
+          "length": 7,
+          "offset": 0,
+        },
+      ],
+      "inlineStyleRanges": Array [],
+      "key": "a",
+      "text": "badlink",
+      "type": "unstyled",
+    },
+  ],
+  "entityMap": Object {},
+}
+`;
+
 exports[`must be able to convert from draft state with ContentBlock to raw 1`] = `
 Object {
   "blocks": Array [

src/model/encoding/__tests__/convertFromDraftStateToRaw-test.js

@@ -61,8 +61,8 @@ const treeContentState = contentState.set(
   ]),
 );
 
-const getMetadata = entityKey =>
-  Immutable.Repeat(CharacterMetadata.create({entity: entityKey}), 5);
+const getMetadata = (entityKey, length) =>
+  Immutable.Repeat(CharacterMetadata.create({entity: entityKey}), length);
 const getLink = entityKey =>
   new DraftEntityInstance({
     type: 'LINK',
@@ -79,23 +79,23 @@ const contentStateWithNonContiguousEntities = ContentState.createFromBlockArray(
       key: 'a',
       type: 'unstyled',
       text: 'link2 link2 link3',
-      characterList: getMetadata('2')
+      characterList: getMetadata('2', 5)
         .toList()
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('2'))
+        .concat(getMetadata('2', 5))
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('3')),
+        .concat(getMetadata('3', 5)),
     }),
     new ContentBlock({
       key: 'b',
       type: 'unstyled',
       text: 'link4 link2 link5',
-      characterList: getMetadata('4')
+      characterList: getMetadata('4', 5)
         .toList()
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('2'))
+        .concat(getMetadata('2', 5))
         .push(CharacterMetadata.EMPTY)
-        .concat(getMetadata('5')),
+        .concat(getMetadata('5', 5)),
     }),
   ],
 )
@@ -104,6 +104,15 @@ const contentStateWithNonContiguousEntities = ContentState.createFromBlockArray(
   .addEntity(getLink('4'))
   .addEntity(getLink('5'));
 
+const contentStateWithEntitiesNotInMap = ContentState.createFromBlockArray([
+  new ContentBlock({
+    key: 'a',
+    type: 'unstyled',
+    text: 'badlink',
+    characterList: getMetadata('999', 7).toList(),
+  }),
+]);
+
 const assertConvertFromDraftStateToRaw = content => {
   expect(convertFromDraftStateToRaw(content)).toMatchSnapshot();
 };
@@ -119,3 +128,7 @@ test('must be able to convert from draft state with ContentBlockNode to raw', ()
 test('must be able to convert from draft state with noncontiguous entities to raw', () => {
   assertConvertFromDraftStateToRaw(contentStateWithNonContiguousEntities);
 });
+
+test('must be able to convert from draft state entities not in map', () => {
+  assertConvertFromDraftStateToRaw(contentStateWithEntitiesNotInMap);
+});

src/model/encoding/convertFromDraftStateToRaw.js

@@ -19,6 +19,7 @@ import type {RawDraftContentState} from 'RawDraftContentState';
 const ContentBlock = require('ContentBlock');
 const ContentBlockNode = require('ContentBlockNode');
 const DraftStringKey = require('DraftStringKey');
+const FBLogger = require('FBLogger');
 
 const encodeEntityRanges = require('encodeEntityRanges');
 const encodeInlineStyleRanges = require('encodeInlineStyleRanges');
@@ -117,12 +118,16 @@ const encodeRawEntityMap = (
   const rawEntityMap = {};
 
   Object.keys(entityMap).forEach((key, index) => {
-    const entity = contentState.getEntity(DraftStringKey.unstringify(key));
-    rawEntityMap[index] = {
-      type: entity.getType(),
-      mutability: entity.getMutability(),
-      data: entity.getData(),
-    };
+    try {
+      const entity = contentState.getEntity(DraftStringKey.unstringify(key));
+      rawEntityMap[index] = {
+        type: entity.getType(),
+        mutability: entity.getMutability(),
+        data: entity.getData(),
+      };
+    } catch (e) {
+      FBLogger('draft-js-contrib').catching(e);
+    }
   });
 
   return {