Fix return value of Source::Importer#import

In SourcesController#import, we rely on the return value of this method
to determine the success or failure of the import. In cases where the
parsing was successful, but no events were found, we were returning a
truthy empty array.
Merge pull request #202 from reidab/source-importer-error-on-no-events

Fix return value of Source::Importer#import