Server needs to respond with more helpful error messages when issuance fails

[bwesterb]

[bwesterb]

We could distinguish between two kind of errors:

• Internal errors that we do not return to the user: eg. failure to write to disk.
• User errors. For instance: certificate revoked. We could return that to the user.

We can also use this distinction for QueueMultiple, where abort early for the former, but just skip assertions with the latter