dri: Emit a critical error if a named driver fails to load.

Something has gone wrong if we were asked to load a driver of a
specific name, but it failed to load for some reason. The user really
should be made aware of this, (and instructed to set LIBGL_DEBUG for
more details).

Reviewed-by: Eugeni Dodonov <eugeni.dodonov@intel.com>

src/glx/dri2_glx.c

@@ -1135,6 +1135,8 @@ dri2CreateScreen(int screen, struct glx_display * priv)
    return &psc->base;
 
 handle_error:
+   CriticalErrorMessageF("failed to load driver: %s\n", driverName);
+
    if (configs)
        glx_config_destroy_list(configs);
    if (visuals)

src/glx/dri_glx.c

@@ -916,6 +916,8 @@ driCreateScreen(int screen, struct glx_display *priv)
    return &psc->base;
 
 cleanup:
+   CriticalErrorMessageF("failed to load driver: %s\n", driverName);
+
    if (psc->driver)
       dlclose(psc->driver);
    glx_screen_cleanup(&psc->base);