Make TxBuilder::get_next_commitment_stats fallible

Anytime we ask `TxBuilder` for stats on a commitment transaction,
`TxBuilder` can now return an error to indicate that a balance not
including the commitment transaction fee has been overdrawn. We then
map this error to the appropriate action depending on where in the
life cycle of the channel the error occurred.

We now do not require that `channel_value_satoshis * 1000` is greater
than or equal to `value_to_holder_msat`; we previously would panic
in this case.

Author: tankyleo