More detailed logging

[pieterdp]

The current logger logs when a record could be submitted (INFO) or when it encountered a failure while processing (ERROR). While this works, it isn't very detailed.

Proposal for enhancements:

• See Exception handling #11 : improve exception handling and logging (don't fail if not really necessary)
• Detailed logging on why a record failed to submit (note: if the underlying Catmandu module isn't helpful, there isn't much we can do)
• Support for statistics: how many records were submitted, how many failed and why