Add guards to some datatypes that fail on recursive calls
Found source of nested config bugs + extended types

* Tweaked cuttlefish_nested_schema_test to provoke the bug more clearly.
* When transform_extended_type encounters an error, it didn't continue
  processing the list. This means that anything that isn't a string and
  isn't of that type would fail and not try any other types. Now we cons
  the error into the accumulator and continue.