mesa: Fix bogus transform feedback error message when subscripting non-array.

mesa: Fix bogus transform feedback error message when subscripting non-array.

Previous to this patch, if the client requested transform feedback
using a subscript, but the variable was not an array
(e.g. "gl_FrontColor[0]"), we would produce a bogus error message like
"Transform feedback varying gl_FrontColor[0] found, but it's an array
([] expected)".

Changed the error message to e.g. "Transfrorm feedback varying
gl_FrontColor[0] requested, but gl_FrontColor is not an array."

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>