Handle ControlledTerm errors in ChangesetErrors.error_details/1

[bmquinn]

• Adds format_error/1 private functions to MeadowWeb.Schema.ChangesetErrors in order to translate {:error, 404} and {:error, :unknown_authority} responses from Authoritex for human readability.
• Adds tests for MeadowWeb.Schema.ChangesetErrors that didn't exist previously.
• Upgrades Authoritex to 0.5.0 to use the new mock prefix missing_id_authority: which forces an {:error, 404} response (though this implementation will likely change soon: see Rewrite Mock to handle known authorities authoritex#43 for more information)