opengl: improve error on pl_scale loading error

When pl_scale could not be loaded, explicitly log an error to report that --gl-upscaler and --gl-downscaler options will be ignored.

In practice, if --gl-upscaler=6 --gl-downscaler=6 and pl_scale could not be loaded, here is the output:

[000060d0000e2ce0] gl gl error: Could not load OpenGL filter 'pl_scale'
[000060d0000e2ce0] gl gl error: Could not apply upscaler filter, ignoring --gl-upscaler=6
[000060d0000e2ce0] gl gl error: Could not apply downscaler filter, ignoring --gl-downscaler=6

However, we don't really know "why" things are not working (maybe the filter is not present, or it failed to load for some reason).

Refs !939 (comment 292729)