[FirebaseObject] Error message is never reset

[kptdobe]

While exploring the "patch" requests (see #321), I realised you cannot test if some properties are present in the FirebaseObject / event. Consider this code:

FirebaseObject event = Firebase.readEvent();

String v = event.getString('nonexistingproperty'); // v = ""
println(event.error()); // prints "failed to convert to string"

v = event.getString('existingproperty'); // v = "value of existingproperty"
println(event.error()); // still prints "failed to convert to string"

The main issue here is that you can only test once if a property exists or not using FirebaseObject#failed() or FirebaseObject#success(): once the error message is set in the FirebaseObject, it is never reset.

[samtstern]

Fixed in #391