glrenderer: Emit more useful error messages

glrenderer: Emit more useful error messages

Catch the error when it happens, so that we can emit a specific and more
helpful error message.

Also verify that all branches in the code now do indeed set a proper
GError when they fail, so that the final catch-all is no longer needed.
Instead, assert that the error is set so that we catch future code
additions early that do not set the GError.