qapi: Move context-sensitive checking to the proper place

qapi: Move context-sensitive checking to the proper place

When we introduced the QAPISchema intermediate representation (commit
ac88219a6c7), we took a shortcut: we left check_exprs() & friends
alone instead of moving semantic checks into the
QAPISchemaFOO.check().  The .check() assert check_exprs() did its job.

Time to finish the conversion job.  Move exactly the context-sensitive
checks to the .check().  They replace assertions there.  Context-free
checks stay put.

Fixes the misleading optional tag error demonstrated by test
flat-union-optional-discriminator.

A few other error message improve.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20190927134639.4284-17-armbru@redhat.com>