Make helper that check for record existence throw informative errors

[asishallab]

Currently the helper functions

• checkExistence and
• assocArgsAreExistingIDs
  return a boolean value.

Change that. The above functions should stop execution immediately if they find an ID in the argument Array that refers to a non existing record. An informative Error should be thrown in this case:
ID ${currId} has no existing record in data model ${model.definition.model}

Change names of the helper functions so they remain informative:

• checkExistence -> validateExistence
• assocArgsAreExistingIDs -> validateAssociationArgsExistence

[asishallab]

Track name change also in our spec.

[tmvoe]

As discussed, checkExistence was renamed as filterOutIdsNotInUse to better match the implementation.