Add a better toString to StreamExection

[marmbrus]

This is just a quick hack, but as I learned today... its nearly impossible to use this in notebooks unless there is a little bit of debugging info provided by the returned object. This is similar to the debugging info we provide in QueryExecution today for batch queries.

=== Streaming Query ===
CurrentOffsets: {}
Thread State: TERMINATED
java.io.FileNotFoundException: File /asdfasdf does not exist
    at org.apache.hadoop.fs.RawLocalFileSystem.listStatus(RawLocalFileSystem.java:379)
    at org.apache.hadoop.fs.FileSystem.listStatus(FileSystem.java:1482)
    at org.apache.hadoop.fs.FileSystem.listStatus(FileSystem.java:1522)
    at org.apache.hadoop.fs.ChecksumFileSystem.listStatus(ChecksumFileSystem.java:564)
    at org.apache.spark.sql.execution.streaming.FileStreamSource.fetchAllFiles(FileStreamSource.scala:123)
    at org.apache.spark.sql.execution.streaming.FileStreamSource.fetchMaxOffset(FileStreamSource.scala:46)
    at org.apache.spark.sql.execution.streaming.FileStreamSource.getNextBatch(FileStreamSource.scala:71)
    at org...

[rxin]

Can you paste a tostring output?

[marmbrus]

Added

[rxin]

LGTM - although it'd be slightly better to not print a blank line in the case of no error

[AmplabJenkins]

Merged build finished. Test PASSed.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/spark-streaming-df-test/46/
Test PASSed.