This commit adds detailed errors for all possible errors returned from
`sendrawtransaction` or `testmempoolaccept`, enabling upstream callers
to have refined control over the actions to be taken.
This commit creates a `RejectReasonMap` to map the errors returned from
`btcd` to bitcoind's `testmempoolaccept` so the `RejectReason` is
unified at the RPC level. To make sure the map keys are unique, the
error strings are modified in `btcd`.
