qapi: Prepare for errors during check()

The next few patches will start migrating error checking from
ad hoc parse methods into the QAPISchema*.check() methods.  But
for an error message to display, we first have to fix the
overall 'try' to catch those errors.  We also want to enable a
few more assertions, such as making sure every attempt to
raise a semantic error is passed a valid location info, or that
various preconditions hold.

The general approach for moving error checking will then be to
relax an assertion into an if that raises an exception if the
condition does not hold, and removing the counterpart ad hoc
check done during the parse phase.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444710158-8723-3-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
