gl_x11: change error message when GL3 context creation fails

On systems that provide legacy OpenGL (up to 2.1), but not GL3 and
later, creating a GL3 context will fail. We then revert to legacy GL.

Apparently the error message printed when the GL3 context creation
fails is confusing. We could just silence it, but there's still a X
error ("X11 error: GLXBadFBConfig"), which would be quite hard to
filter out. For one, it would require messing with the X11 error
handler, which doesn't even carry a context pointer (for application
private data), so we don't even want to touch it. Instead, change
the error message to inform the user what's actually happening: a
fallback to an older version of OpenGL.