Mongo Sink v0.4.2

Error message improvements.

Changed

• This version changes how errors are handled to improve error messages.
  Errors now show more context and additional information that will help
  developers debug their indexers.
• The sink will return a non-zero error code on failure. We use the standard
  unix exit codes in sysexit.h. Developers can use exit codes to decide
  whether to restart the indexer or not.