Fix the clippy::manual_non_exhaustive warning

```
warning: this seems like a manual implementation of the non-exhaustive pattern
  --> src/error.rs:11:1
   |
11 |   pub enum Error {
   |   ^-------------
   |   |
   |  _help: add the attribute: `#[non_exhaustive] pub enum Error`
   | |
12 | |     ParseError {
13 | |         msg: String,
14 | |         line: usize,
...  |
45 | |     __NonExhaustive,
46 | | }
   | |_^
   |
   = note: `#[warn(clippy::manual_non_exhaustive)]` on by default
help: remove this variant
  --> src/error.rs:45:5
   |
45 |     __NonExhaustive,
   |     ^^^^^^^^^^^^^^^
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#manual_non_exhaustive

warning: 1 warning emitted
```

Signed-off-by: Nathaniel McCallum <[email protected]>

Author: npmccallum

src/error.rs

@@ -8,6 +8,7 @@ use std::fmt;
 
 /// Enum representing the potential errors that TinyTemplate can encounter.
 #[derive(Debug)]
+#[non_exhaustive]
 pub enum Error {
     ParseError {
         msg: String,
@@ -40,9 +41,6 @@ pub enum Error {
         line: usize,
         column: usize,
     },
-
-    #[doc(hidden)]
-    __NonExhaustive,
 }
 impl From<SerdeJsonError> for Error {
     fn from(err: SerdeJsonError) -> Error {
@@ -102,7 +100,6 @@ impl fmt::Display for Error {
                     name, line, column, err
                 )
             }
-            Error::__NonExhaustive => unreachable!(),
         }
     }
 }
@@ -116,7 +113,6 @@ impl StdError for Error {
             Error::StdFormatError { .. } => "StdFormatError",
             Error::CalledTemplateError { .. } => "CalledTemplateError",
             Error::CalledFormatterError { .. } => "CalledFormatterError",
-            Error::__NonExhaustive => unreachable!(),
         }
     }
 }