[stable-34.0] fix(cmd,csync): don't silently ignore a missing --exclude file - #10291

nextcloudcmd's ExcludedFiles::reloadExcludeFiles() dropped a
registered exclude file from the list without any warning when
QFile::exists() returned false, and still reported success. A
mistyped or unresolved --exclude path (e.g. a relative path that
resolves differently under cron than interactively) therefore made
nextcloudcmd sync everything with zero exclusions and no diagnostic
in the log.

- csync_exclude.cpp: log a qWarning when a registered exclude file
  can't be found, instead of silently erasing it.
- cmd.cpp: fail fast with qFatal when a user-supplied --exclude path
  doesn't exist, instead of only discovering it deep inside
  reloadExcludeFiles() with no way to distinguish "path is wrong"
  from "path was never wrong to begin with".

Fixes #4621

Signed-off-by: mosandlt <[email protected]>