Improve error message if starting picom with a backend missing compile support

[bubbleguuum]

Picom v11.2

If you start picom with --backend glx with picom not compiled with OpenGL, you get this error:

# [ 04/16/24 23:04:42.839 get_cfg ERROR ] Backend "glx" is only available as part of the legacy backends.
# [ 04/16/24 23:04:42.839 session_init FATAL ERROR ] Failed to get configuration, usually mean you have specified invalid options.
# [ 04/16/24 23:04:42.839 main FATAL ERROR ] Failed to create new session.

It would be less confusing to have an explicit error about picom not being compiled with OpenGL support .
Same thing with the EGL backend also requiring OpenGL.