[SPARK-1688] Propagate PySpark worker stderr to driver

When at least one of the following conditions is true, PySpark cannot be loaded:

1. PYTHONPATH is not set
2. PYTHONPATH does not contain the python directory (or jar, in the case of YARN)
3. The jar does not contain pyspark files (YARN)
4. The jar does not contain py4j files (YARN)

However, we currently throw the same random `java.io.EOFException` for all of the above cases, when trying to read from the python daemon's output. This message is super unhelpful.

This PR includes the python stderr and the PYTHONPATH in the exception propagated to the driver. Now, the exception message looks something like:

```
Error from python worker:
  : No module named pyspark
PYTHONPATH was: 
  /path/to/spark/python:/path/to/some/jar
java.io.EOFException
  <stack trace>
```

whereas before it was just

```
java.io.EOFException
  <stack trace>
```