refactor: Error message bilingual_str consistency

- Move the decision whether to translate an error message to where it is
  defined. This simplifies call sites: no more `InitError(Untranslated(...))`.

- Make all functions in `util/error.h` consistently return a
  `bilingual_str`. We've decided to use this as error message type so
  let's roll with it.

This has no functional changes: no messages are changed, no new
translation messages are defined.

src/rpc/util.cpp

@@ -10,6 +10,7 @@
 #include <tinyformat.h>
 #include <util/strencodings.h>
 #include <util/string.h>
+#include <util/translation.h>
 
 #include <tuple>
 
@@ -285,7 +286,7 @@ UniValue JSONRPCTransactionError(TransactionError terr, const std::string& err_s
     if (err_string.length() > 0) {
         return JSONRPCError(RPCErrorFromTransactionError(terr), err_string);
     } else {
-        return JSONRPCError(RPCErrorFromTransactionError(terr), TransactionErrorString(terr));
+        return JSONRPCError(RPCErrorFromTransactionError(terr), TransactionErrorString(terr).original);
     }
 }