Consolidate NMClientError and NMObjectError (such that there is now
only one libnm-API-specific error domain). In particular, merge
NM_CONNECTION_ERROR_CONNECTION_REMOVED with
NM_OBJECT_ERROR_OBJECT_CREATION_FAILURE as the new
NM_CONNECTION_ERROR_OBJECT_CREATION_FAILED.

Also make object_creation_failed() be a plain method rather than a
signal, since there's no reason for anyone to be connecting to it on
another object. And remove its GError argument because the subclass
can just create its own more-specific error.