[build] Add several util units

Adds the following util units and adds them to libbitcoin_util:

- `util/url.cpp` takes `urlDecode` from `httpserver.cpp`
- `util/error.cpp` takes `TransactionErrorString` from
  `node/transaction.cpp` and `AmountHighWarn` and `AmountErrMsg` from
  `ui_interface.cpp`
- `util/fees.cpp` takes `StringForFeeReason` and `FeeModeFromString` from `policy/fees.cpp`
- `util/rbf.cpp` takes `SignalsOptInRBF` from `policy/rbf.cpp`
- 'util/validation.cpp` takes `FormatStateMessage` and `strMessageMagic` from 'validation.cpp`

src/util/error.h

@@ -0,0 +1,38 @@
+// Copyright (c) 2010-2018 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_UTIL_ERROR_H
+#define BITCOIN_UTIL_ERROR_H
+
+/**
+ * util/error.h is a common place for definitions of simple error types and
+ * string functions. Types and functions defined here should not require any
+ * outside dependencies.
+ *
+ * Error types defined here can be used in different parts of the bitcoin
+ * codebase, to avoid the need to write boilerplate code catching and
+ * translating errors passed across wallet/node/rpc/gui code boundaries.
+ */
+
+#include <string>
+
+enum class TransactionError {
+    OK, //!< No error
+    MISSING_INPUTS,
+    ALREADY_IN_CHAIN,
+    P2P_DISABLED,
+    MEMPOOL_REJECTED,
+    MEMPOOL_ERROR,
+    INVALID_PSBT,
+    PSBT_MISMATCH,
+    SIGHASH_MISMATCH,
+};
+
+std::string TransactionErrorString(const TransactionError error);
+
+std::string AmountHighWarn(const std::string& optname);
+
+std::string AmountErrMsg(const char* const optname, const std::string& strValue);
+
+#endif // BITCOIN_UTIL_ERROR_H