Call node->initError instead of InitError from GUI code

Avoids GUI code calling a node function, and having to live in the same process
as g_ui_signals and uiInterface global variables.

src/interfaces/node.h

@@ -38,6 +38,9 @@ class Node
 public:
     virtual ~Node() {}
 
+    //! Send init error.
+    virtual void initError(const std::string& message) = 0;
+
     //! Set command line arguments.
     virtual bool parseParameters(int argc, const char* const argv[], std::string& error) = 0;