Improve error messages when typeToCode throws an exception

[fromkeith]

typeToCode can throw an expection, which reduces the error message to:

UnimplementedError: (InvalidTypeImpl) InvalidType
package:json_serializable/src/utils.dart 221:3              typeToCode
package:json_serializable/src/helper_core.dart 79:46        createInvalidGenerationError
...

This change improves the error message to something like this:

Could not generate `fromJson` code for `photos` because of type is unimplemented/unsupported/undefined.
package:picthrive_kiosk/ptclient/generated/consolidated_public.swagger.dart:2633:21
     ╷
2633 │   final List<Items> photos;
     │                     ^^^^^^
     ╵

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