glsl: Handle failure of Python codegen scripts.
glsl: Handle failure of Python codegen scripts.

If a Python codegen script failed, it would write a zero-byte file,
which on subsequent invocations of make would trick it into thinking the
file was appropriately generated.

Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler