Better error handling for '-layout' option

[kasemir]

The '-layout' option introduced in #1747 works, but the PhoebusApplication will log "Ignoring launch parameter '-layout'" even when the option is handled fine.
If the file does not exist, the '-layout' option silently ignores it.

Should print no warning on success, but do warn when the file cannot be located.

[katysaintin]

I also add logging informations for this task in the PR #3250
Logs will displayed if the layout exists and if yes where Phoebus find it.
It will warn if the layout not exist and the layout option may be used in settings.ini also