JERSEY-728: Multipart unmarshalling should throw exceptions instead of returning null when it fails.

 - Fix (throwing ProcessingException instead of doing nothing when IOException occurs).
 - Added test case.
 - Refactorings (localization, javadocs, ..).

Change-Id: I3f4a346b67f61c0cb517fe25bf2df716ac6316f1
Merge "JERSEY-728: Multipart unmarshalling should throw exceptions instead of returning null when it fails."