Throw exception instead of questionable `EntitySource.safeMapGet`

[sebastian-peter]

Once #773 is merged:

The whole procedure within EntitySource.safeMapGet seems somewhat questionable to me. We rather want to throw an exception if a required value does not exist, instead of returning some error message as a String in place of the desired value. For non-required values (if applicable), one can just handle the result of Optional.ofNullable(map.get(key)).

Originally mentioned by @sebastian-peter in #773 (comment)

[sebastian-peter]

Ok, there was actually an understandable reason for this method to exist: #933 (review)