Change error_message from wstring to string

This removes a lot of narrow/wide conversions where a wide string was never used.

src/camera.h

@@ -110,7 +110,7 @@ public:
 	}
 
 	// Checks if the constructor was able to create the scene nodes
-	bool successfullyCreated(std::wstring& error_message);
+	bool successfullyCreated(std::string &error_message);
 
 	// Step the camera: updates the viewing range and view bobbing.
 	void step(f32 dtime);