GL: Use compile/link status for errors

[CallumDev]

Fixes demo crashing when the shader compiler reports warnings in the info log.

Example of previous error on Linux:

Unhandled exception. System.Exception: 0:49(38): warning: `_189' used uninitialized
0:49(51): warning: `_192' used uninitialized
0:49(64): warning: `_194' used uninitialized

[RossNordby]

(quickly stopping in to note that I've seen this and thanks! I continue to be unusually swamped and excessively travel prone, but I'm hoping next week is less dumb.)