Validator: handle undefined observables more gracefully

[dweindl]

What happens if measurement table contains an observableId that is not defined in the observables table:

  File "PEtab/petab/lint.py", line 127, in check_measurement_df
    trafo = observable_df.loc[obs_id, OBSERVABLE_TRANSFORMATION]
  File "lib/python3.7/site-packages/pandas/core/indexing.py", line 1761, in __getitem__
    return self._getitem_tuple(key)
  File "lib/python3.7/site-packages/pandas/core/indexing.py", line 1271, in _getitem_tuple
    return self._getitem_lowerdim(tup)
  File "lib/python3.7/site-packages/pandas/core/indexing.py", line 1388, in _getitem_lowerdim
    section = self._getitem_axis(key, axis=i)
  File "lib/python3.7/site-packages/pandas/core/indexing.py", line 1964, in _getitem_axis
    return self._get_label(key, axis=axis)
  File "lib/python3.7/site-packages/pandas/core/indexing.py", line 624, in _get_label
    return self.obj._xs(label, axis=axis)
  File "lib/python3.7/site-packages/pandas/core/generic.py", line 3537, in xs
    loc = self.index.get_loc(key)
  File "lib/python3.7/site-packages/pandas/core/indexes/base.py", line 2648, in get_loc
    return self._engine.get_loc(self._maybe_cast_indexer(key))
  File "pandas/_libs/index.pyx", line 111, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 138, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 1618, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1626, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'K27me0K36me0'

What should have happened:
A more informative message should be printed (and potentially validation should continue)