[ThorChainSwap/BTC] Handle memo-overflow cases as error

[optout21]

ThorChainSwap uses memo field for X-chain swap info. In case of swap from Bitcoin chain, memo is stored in OP_RETURN output data. This has a max limit of 80 bytes. In case of longer memo, it is truncated. Instead, the input TX generation should fail with appropriate message, preventing accidental mis-swap. The possibility of recently-introduce affiliate fee generally makes the memo longer (#2715).