Handle the error response when the record is uploaded

Fix edina/fieldtrip-gb#128
Show specific error message for dropbox errors

Issue edina/fieldtrip-gb#128
Finish succesfully if the record doesn't have assets
Add default values to the finish callback

Issue edina/fieldtrip-gb#128
Merge pull request #10 from edina/ftgb-1.4

Handle properly the error response when the error comes from dropbox