qapi: Improve reporting of invalid 'if' further

qapi: Improve reporting of invalid 'if' further

check_if()'s errors don't point to the offending part of the
expression.  For instance:

    tests/qapi-schema/alternate-branch-if-invalid.json:2: 'if' condition ' ' makes no sense

Other check_FOO() do, with the help of a @source argument.  Make
check_if() do that, too.  The example above improves to:

    tests/qapi-schema/alternate-branch-if-invalid.json:2: 'if' condition ' ' of 'data' member 'branch' makes no sense

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190927134639.4284-23-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>