Handle unknown enum values

[samcook]

Currently if the server returns a value for an enum that isn't recognised, the (objective-C) fickle code will default the enum to the first value.

Possible solutions:

1. Throw an exception if an unknown enum value is encountered. (Downside of this approach is that, if the message contains a list of objects and one object fails due to an invalid enum, the entire message would be discarded).
2. Automatically generate an "unknown" value on the client side for all enums. (This approach would allow the client to decide what to do on a per enum basis).