Backport #16257.
Cherry-picked from the 0.18 backport in #16414, but without the [wip] messages and without the last commit (which adds a test in a file that didn't exist in 0.17).
Concept ACK.
FundTransaction calls GetMinimumFee which, when the fee rate is absurdly high, quietly reduced the fee to -maxtxfee. Becaue an absurdly high fee rate is usually the result of a fat finger, aborting seems safer behavior.
Github-Pull: #16257
Rebased-From: 806b0052c3b45415862f74f20ba5f389e5b673de
Github-Pull: #16322
Rebased-From: 5c1b9714cb0a13be28324f91f4ec9ca66a1de8c7
Manually added Github-Pull: and Rebased-From fields.
ACK 0e7c7465bf2b9911da094d9790480f94e158f9a5
Thanks. Note that 0.18 backport is still open in https://github.com/bitcoin/bitcoin/pull/16414