qapi: Share gen_err_check()

qapi-commands has a nice helper gen_err_check(), but did not
use it everywhere. In fact, using it in more places makes it
easier to reduce the lines of code used for generating error
checks.  This in turn will make it easier for later patches
to consolidate another common pattern among the generators.

The generated code has fewer blank lines in qapi-event.c functions,
but has no semantic difference.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1443565276-4535-16-git-send-email-eblake@redhat.com>
[Drop another blank line for symmetry]
Signed-off-by: Markus Armbruster <armbru@redhat.com>