`os.path.exists("/run/weldr/api.socket")` returns False for users which have no
access. This leads to composer printing that the file does not exist, which is
misleading.
Since it's no possible to distinguish the two cases, fix this problem by
combining them and showing a single error message.
Remove `except` block which immediately raises the same exception again (it's
not a subclass of another caught exception, so this is safe).
Remove a false positive, because it is not emitted from the code base.
Disable subprocess-popen-preexec-fn in startProgram, which is not used
internally.
