HDFS throws FileNotFoundException on getFileBlockLocations(path-to-directory)

The HDFS implementation of getFileBlockLocations(path, offset, len) throws an exception if the path references a directory.

The base implementation (and all other filesystems) just return an empty array, something implemented in getFileBlockLocations(filestatsus, offset, len); something written up in filesystem.md as the correct behaviour.

1. has been shown to break things: SPARK-14959
2. there's no contract tests for these APIs; shows up in HADOOP-15044.
3. even if this is considered a wontfix, it should raise something like PathIsDirectoryException rather than FNFE