[COMMIT] Specify ERROR_ME_WARN explicitly in specifier_instance_from_inst_list().

[Stephen J. Turnbull]

Aidan Kehoe writes:
 > 
 > Giacomo, in my local tests, this fixes the gnuclient problem you were
 > seeing. There is still a warning about the attempt to instantiate a pixmap
 > specifier on a TTY; this warning is probably wrong, and I’ll fix it
 > at some point, but this should be good enough for most purposes
 > now.

It's not obvious to me that the warning is wrong.  Pixmaps probably
"should" only be specified with a tag-set meaning "graphical display".

OTOH, it's reasonable for DWIM functions like specifier-instance and
set-specifier to try to do something about this.  In the case of
specifier-instance, maybe it's OK to simply suppress the warning.
set-specifier OTOH might want to helpfully add such a tag-set.