[SPARK-1466] Raise exception if pyspark Gateway process doesn't start.

If the gateway process fails to start correctly (e.g., because JAVA_HOME isn't set correctly, there's no Spark jar, etc.), right now pyspark fails because of a very difficult-to-understand error, where we try to parse stdout to get the port where Spark started and there's nothing there. This commit properly catches the error and throws an exception that includes the stderr output for much easier debugging.

Thanks to @shivaram and @stogers for helping to fix this issue!

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #383 from kayousterhout/pyspark and squashes the following commits:

36dd54b [Kay Ousterhout] [SPARK-1466] Raise exception if Gateway process doesn't start.
SPARK-2158 Clean up core/stdout file from FileAppenderSuite

@tdas

Author: Mark Hamstra <markhamstra@gmail.com>

Closes #1100 from markhamstra/SPARK-2158 and squashes the following commits:

ae8e069 [Mark Hamstra] Response to TD's review
2f1e201 [Mark Hamstra] Cleanup 'stdout' file within FileAppenderSuite
Remove unicode operator from RDD.scala

Some IDEs don’t support unicode characters in source code. Check if this breaks binary compatibility.

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1119 from dorx/unicode and squashes the following commits:

05618c3 [Doris Xin] Remove unicode operator from RDD.scala
[SPARK-2184][SQL] AddExchange isn't idempotent

...redPartitioning.

Author: Michael Armbrust <michael@databricks.com>

Closes #1122 from marmbrus/fixAddExchange and squashes the following commits:

3417537 [Michael Armbrust] Don't bind partitioning expressions as that breaks comparison with requiredPartitioning.
Squishing a typo bug before it causes real harm

in updateNumRows method in RowMatrix

Author: Doris Xin <doris.s.xin@gmail.com>

Closes #1125 from dorx/updateNumRows and squashes the following commits:

8564aef [Doris Xin] Squishing a typo bug before it causes real harm
[SPARK-2187] Explain should not run the optimizer twice.

@yhuai @marmbrus @concretevitamin

Author: Reynold Xin <rxin@apache.org>

Closes #1123 from rxin/explain and squashes the following commits:

def83b0 [Reynold Xin] Update unit tests for explain.
a9d3ba8 [Reynold Xin] [SPARK-2187] Explain should not run the optimizer twice.
Add describe command.
Merge remote-tracking branch 'upstream/master' into SPARK-2177