The fix for !161 moved updating credentials to reasonably deep within
the IMAP service to fix some poor behaviour for GOA accounts, however
local accounts would still prompt for a password if not present in
libsecret from there, and if cancelled would prompt as many times as the
service attempted to re-open the pool.

This removes the call to prompt for a password from the libsecret
mediator, brings back some of the API removed by commit c8f5e029, so
that services can tell if credentials weren't loaded, and hence notify
appropriately so the controller can then do the prompting.