Previously we would check the first byte only when trying to figure out
the predicate for the second byte. If the first byte was invalid, we'd
then throw with a helpful error message. However this made
wellFormedByte a very weird function.
At the expense of doing the same check twice, we now check the first
byte, when it is first passed, and always return a boolean.
Change-Id: I32ab6051c844711849e5b4a115e2511b53682baa
0e9c770972
