Sanity check channel balance prior to swap

[wtogami]

Prior to starting a swap, and again prior to sending, check the channel's max htlc limit to verify that the requested swap would actually be possible. Print informative error messages otherwise.

Related: #73

[nepet]

Why

So this issue is a little bit paranoid but we want to do everything we can to avoid canceled swaps, especially swap-out swaps as the responder is in charge of the on-chain part.
The following assumes a swap-out but same applies symmetrically for a swap-in:
Currently we only check that we have enough local balance (our_amount_msat) to perform the swap. This check is not strong enough as we could have set a htlc_maximum_msat that is below our local balance.

Solution

We want to check that min(htlc_maximum_msat, our_amount_msat) > swap_amt_msat + fees
Best place to check that would be after we received the swap_out_agreement that contains the "fee_invoice". We may still want to perform the same check without the fees before we initiate a swap.

[wtogami]

It would be nice to fix this because it would prevent guaranteed-to-fail onchain transactions and spamming the swap database.