i965: Modify some error messages to refer to "vec4" instead of "vs".

i965: Modify some error messages to refer to "vec4" instead of "vs".

These messages are in code that is shared between the VS and GS
back-ends, so use the terminology "vec4" to avoid confusion.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>