Add toString to payment part failures

[pm47]

Follow-up to #634.

This makes FinalFailure.explain() return consistent results.

[t-bast]

It was on purpose that this doesn't have a toString(), since wallet applications must provide their own localized string for each failure (this is in the comment on that trait). Why do you need it?

[pm47]

PaymentFailure also must be localized, but we still provide human-readable toStrings.

I'm trying to avoid doing this on phoenixd: ACINQ/phoenixd@27799db