Print out more friendly error if listFiles() fails

listFiles() could return null if the I/O fails, and this currently results in an ugly NPE which is hard to diagnose.
Style fix: put body of if within curly braces
Merge pull request #915 from ooyala/master

Get rid of / improve ugly NPE when Utils.deleteRecursively() fails
Document fortran dependency for MLBase
Merge pull request #917 from pwendell/master

Document libgfortran dependency for MLBase
Update Python API features
Fixing SPARK-602: PythonPartitioner

Currently PythonPartitioner determines partition ID by hashing a byte-array
representation of PySpark's key. This PR lets PythonPartitioner use the
actual partition ID, which is required e.g. for sorting via PySpark.