glsl: Slightly restructure error generation in validate_explicit_location

glsl: Slightly restructure error generation in validate_explicit_location

Use mode_string to get the name of the variable mode.  Slightly change
the control flow.  Both of these changes make it easier to support
separate shader object location layouts.

The format of the message changed because mode_string can return a
string like "shader output".  This would result in an awkward message
like "vertex shader shader output..."

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>