Today, the error message is only constructed when it's used. This commit
changes qerror to construct the error message when the error object is
built (ie. when the error is reported).

This eliminates the need of storing a pointer to qerror_table[], which
will be dropped soon, and also simplifies the code.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>