charmloader: log fails to file

Log all charms and bundles that failed the ingestion process. With err descriptions if possible.
I'm not sure that the custom writer is necessary.
How about just doing this:

    err := loggo.RegisterWriter("errors", writer, loggo.ERROR)

?
Then any log messages of error and above will get written
to this file, which I think is what we want here.

Note - as written, the condition below would never get
triggered, because err wasn't being assigned.