qapi: Fix broken discriminator error messages

qapi: Fix broken discriminator error messages

check_union() checks the discriminator exists in base and makes sense.
Two error messages mention the base.  These are broken for anonymous
bases, as demonstrated by tests flat-union-invalid-discriminator and
flat-union-invalid-if-discriminator.err.  The third one doesn't
bother.

First broken when commit ac4338f8eb "qapi: Allow anonymous base for
flat union" (v2.6.0) neglected to adjust the "not a member of base"
error message.  Commit ccadd6bcba "qapi: Add 'if' to implicit struct
members" (v4.0.0) then cloned the flawed error message.

Dumb them down not to mention the base.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190914153506.2151-11-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>