Fix error logging exception message
Fix error logging exception message

Fix error encountered in logging an exception's message in
Encoder::create_media. The variable `model` is an ActiveRecord model,
not a string, and does not have a `#capitalize` method. `type.to_s`
does, and gets more to the point, anyway.

dpla/primary-source-sets,dpla/primary-source-sets,dpla/primary-source-sets