Fix no-op transaction error on paste.create Conduit API method

Summary:
Fixes T9735. I changed how the TYPE_LANGUAGE transction works a little but that accidentally tripped an error condition in paste.create.

• Don't bail on no-effect transactions to paste.create (like not setting a language).
• When a transaction type has no tailored UI message, make it easier to figure out which transaction is problematic.

Test Plan: Ran arc paste ... locally. Got an error before the patch, clean paste creation afterward.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9735

Differential Revision: https://secure.phabricator.com/D14440