Error when SubarrayDescription.select_subarray is given unknown telescope ids could be better

[aknierim]

When using more than one allowed telescope in your config file and the pedestal dataset only contains data for a specific telescope id you get the following, imprecise error message:

ERROR [ctapipe.ctapipe-process] (tool.run): Caught unexpected exception: 2
Traceback (most recent call last):
File "/ctasoft/ctapipe/ctapipe/core/tool.py", line 336, in run
    self.setup()
  File "/ctasoft/ctapipe/ctapipe/tools/process.py", line 150, in setup
    self.event_source = EventSource(parent=self)
  File "/ctasoft/ctapipe/ctapipe/io/simteleventsource.py", line 243, in __init__
    self._subarray_info = self.prepare_subarray_info(
  File "/ctasoft/ctapipe/ctapipe/io/simteleventsource.py", line 364, in prepare_subarray_info
    subarray = subarray.select_subarray(self.allowed_tels)
  File "/ctasoft/ctapipe/ctapipe/instrument/subarray.py", line 319, in select_subarray
    tel_positions = {tid: self.positions[tid] for tid in tel_ids}
  File "/ctasoft/ctapipe/ctapipe/instrument/subarray.py", line 319, in <dictcomp>
    tel_positions = {tid: self.positions[tid] for tid in tel_ids}
KeyError: 2

An error message telling the user what telescope id's are allowed would be helpful for future use.