[wallet] abort when attempting to fund a transaction above maxtxfee

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.

src/util/error.cpp

@@ -27,6 +27,8 @@ std::string TransactionErrorString(const TransactionError err)
             return "PSBTs not compatible (different transactions)";
         case TransactionError::SIGHASH_MISMATCH:
             return "Specified sighash value does not match existing value";
+        case TransactionError::MAX_FEE_EXCEEDED:
+            return "Fee exceeds maximum configured by -maxtxfee";
         // no default case, so the compiler can warn about missing cases
     }
     assert(false);