Return correct error codes in fundrawtransaction().

The fundrawtransaction() RPC was returning misleading or incorrect error
codes (for example RPC_INTERNAL_ERROR when funding the transaction
failed). This commit fixes those error codes:

- RPC_INTERNAL_ERROR should not be returned for application-level
errors, only for genuine internal errors such as corrupted data.

That error code has been replaced with RPC_WALLET_ERROR.

This commit also updates the test cases to explicitly test the error code.

src/wallet/rpcwallet.cpp

@@ -2761,7 +2761,7 @@ UniValue fundrawtransaction(const JSONRPCRequest& request)
     string strFailReason;
 
     if (!pwallet->FundTransaction(tx, nFeeOut, overrideEstimatedFeerate, feeRate, changePosition, strFailReason, includeWatching, lockUnspents, setSubtractFeeFromOutputs, reserveChangeKey, changeAddress)) {
-        throw JSONRPCError(RPC_INTERNAL_ERROR, strFailReason);
+        throw JSONRPCError(RPC_WALLET_ERROR, strFailReason);
     }
 
     UniValue result(UniValue::VOBJ);