Catch 500 errors on ingest

[jcoyne]

When calling RestApiClient#ingest we get an error: "500 Internal Server Error"

debugging indicates the server is giving this message:
The PID 'hydrangea:fixture_mods_article1' already exists in the registry; the object can't be re-created.

But it just raises a 500 exception up to the application level. We can probably throw something more helpful.

[cbeer]

Agreed. There are likely a number of Fedora errors that Rubydora could rethrow better.