Improve the error handling in the gather and import stage

[metaodi]

• Raise exceptions when parse_dataset in profiles fails
• Do not update content hash if download errored
• Call _save_gather_error if datasets() generators fails

[metaodi]

I added the traceback as we most likely have an error in a custom profile, so this makes the it easier pinpoint the source of the error (in my case I was hunting a KeyError somewhere in my custom profile)

[metaodi]

I added two test cases to test the new error handling

1. The user-contributed profiles are used (parse_dataset), hence we don't have control over the code. If an exception is raised, the harvester should fail gracefully incl. a nice error msg in the log
2. The IDCATRDFHarvester provides extension points to the harvester. As we don't have control over the code behind, the same as above applies: if an exception is raised, the harvester should fail gracefully (possibly only for one harvest_object) and log an error msg.

[amercader]

Thanks @metaodi!