mesa: fix error handling in _mesa_TransformFeedbackVaryings

mesa: fix error handling in _mesa_TransformFeedbackVaryings

Patch changes function to use _mesa_lookup_shader_program_err both
in TransformFeedbackVaryings and GetTransformFeedbackVarying that
handles errors correctly for invalid values of shader program.

Fixes following dEQP test:
   dEQP-GLES31.functional.debug.negative_coverage.get_error.shader.transform_feedback_varyings

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98135
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>