More error logging to launcher backends

There is practically no logging currently in the three different launcher backends: launcher-logind, launcher-weston-launch, and launcher-direct. When they all fail, we get a generic error message telling the used to use weston-launch or logind, but that is not helpful when they already did, especially when logind is supposed to be there but weston still fails.

Add log messages to indicate launcher backend failures with reasons when possible.

A follow-up task idea: add a debug scope that records all interactions with logind.