wallet: return and display signer error

Both RPC and GUI now render a useful error message instead of (silently) failing.

Replace bool with util::Result<void> to clarify that this either succeeds or returns an error message.

src/interfaces/wallet.h

@@ -127,7 +127,7 @@ public:
     virtual bool setAddressReceiveRequest(const CTxDestination& dest, const std::string& id, const std::string& value) = 0;
 
     //! Display address on external signer
-    virtual bool displayAddress(const CTxDestination& dest) = 0;
+    virtual util::Result<void> displayAddress(const CTxDestination& dest) = 0;
 
     //! Lock coin.
     virtual bool lockCoin(const COutPoint& output, const bool write_to_db) = 0;