[voicecall] Fix error passing from providers. Contributes to NEMO#246.

Providers were successfully checking for errors and emitting them, but the
manager ignored their plaintative cries for help and did absolutely nothing,
so the UI never heard about their plight and did nothing while the user
wondered why their phone was not making the ordered phone call for 5 minutes
before throwing it out the window.

Errors are still rather broken:
1) they are not i18n-friendly (hardcoded english)
2) they are not user-friendly ("org.freedesktop.Telepathy.Error.Disconnected:
   Channel aborted")

but at least they are passed up successfully.
[voicecall] Optionally enable Telepathy debug output if WANT_TRACE is defined.
[voicecall] Correct debug output in the error case for dbus pending call watcher.
[voicecall] Remove dependency on xorg.target from voicecall-manager.

This won't work in a Wayland-world, and we also don't require anything
graphical, so we can start whenever.

This assumes that voicecall-ui implementations have a requirement on
voicecall-manager.service.