Meson 0.50: Follow-up from "Meson: Better messages for most dependency errors"

When Weston is ready to start depending on Meson 0.50, which adds not_found_message argument to dependency(), convert as many human-friendly missing dependency error messages to not_found_message as possible.

The discussion from !75 (merged):

• @daniels started a discussion: (+2 comments)

  @whot writes from !75 (merged):

  LGTM, but I wonder if this is something that may be best in meson directly as an argument to dependency(). Seems useful for many cases.

  @pq writes from !75 (merged):

  There is https://github.com/mesonbuild/meson/issues/2407 but I think the messages are a major usability improvement, which I think is crucial when pushing people towards Meson, so I'm not sure we want to wait for a Meson release. Do we?

  Sounds reasonable, and I'm happy to merge this as-is. I'll leave this as a discussion: you can select the option to open issues for all unresolved discussions, so we automatically have a follow-on issue to resolve this when we come to bump our Meson dep to 0.50.