Need to expose verification errors to callers

[jaxley]

Currently, the implementation generates fairly-detailed verification errors objects based on what verification failed. But the current API keeps those entirely internal. This prevents callers from determining what caused an authorization failure.

A common use case would be that a server would need to be able to communicate to a caller that a macaroon or discharge macaroon has expired and they need to obtain a new one. That's opaque right now because .isValid and .assertValid only generate booleans.

Debugging is also a challenge since I've had to put breakpoints in the code in order to determine which failure occurred (whether expected or not). Would also help unit testability to be able to ensure that validations occur because of expected conditions and not coincidence or serendipity.