[SPARK-9393] [SQL] Fix several error-handling bugs in ScriptTransform operator

SparkSQL's ScriptTransform operator has several serious bugs which make debugging fairly difficult:

- If exceptions are thrown in the writing thread then the child process will not be killed, leading to a deadlock because the reader thread will block while waiting for input that will never arrive.
- TaskContext is not propagated to the writer thread, which may cause errors in upstream pipelined operators.
- Exceptions which occur in the writer thread are not propagated to the main reader thread, which may cause upstream errors to be silently ignored instead of killing the job.  This can lead to silently incorrect query results.
- The writer thread is not a daemon thread, but it should be.

In addition, the code in this file is extremely messy:

- Lots of fields are nullable but the nullability isn't clearly explained.
- Many confusing variable names: for instance, there are variables named `ite` and `iterator` that are defined in the same scope.
- Some code was misindented.
- The `*serdeClass` variables are actually expected to be single-quoted strings, which is really confusing: I feel that this parsing / extraction should be performed in the analyzer, not in the operator itself.
- There were no unit tests for the operator itself, only end-to-end tests.

This pull request addresses these issues, borrowing some error-handling techniques from PySpark's PythonRDD.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #7710 from JoshRosen/script-transform and squashes the following commits:

16c44e2 [Josh Rosen] Update some comments
983f200 [Josh Rosen] Use unescapeSQLString instead of stripQuotes
6a06a8c [Josh Rosen] Clean up handling of quotes in serde class name
494cde0 [Josh Rosen] Propagate TaskContext to writer thread
323bb2b [Josh Rosen] Fix error-swallowing bug
b31258d [Josh Rosen] Rename iterator variables to disambiguate.
88278de [Josh Rosen] Split ScriptTransformation writer thread into own class.
8b162b6 [Josh Rosen] Add failing test which demonstrates exception masking issue
4ee36a2 [Josh Rosen] Kill script transform subprocess when error occurs in input writer.
bd4c948 [Josh Rosen] Skip launching of external command for empty partitions.
b43e4ec [Josh Rosen] Clean up nullability in ScriptTransformation
fa18d26 [Josh Rosen] Add basic unit test for script transform with 'cat' command.