all: Get rid of fatal logging (#5537)

* cleanup Fatal in lib/config/config.go

* cleanup Fatal in lib/config/folderconfiguration.go

* cleanup Fatal in lib/model/model.go

* cleanup Fatal in cmd/syncthing/monitor.go

* cleanup Fatal in cmd/syncthing/main.go

* cleanup Fatal in lib/api

* remove Fatal methods from logger

* lowercase in errors.Wrap

* one less channel
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f