Added error messages.

src/fullscreen.c

@@ -110,9 +110,18 @@ GLFWAPI int glfwGetVideoModes(GLFWvidmode* list, int maxcount)
         return 0;
     }
 
-    if (maxcount <= 0 || list == NULL)
+    if (maxcount <= 0)
     {
-        // TODO: Figure out if this is an error
+        _glfwSetError(GLFW_INVALID_VALUE,
+                      "glfwGetVideoModes: Parameter 'maxcount' must be "
+                      "greater than zero");
+        return 0;
+    }
+
+    if (list == NULL)
+    {
+        _glfwSetError(GLFW_INVALID_VALUE,
+                      "glfwGetVideoModes: Parameter 'list' cannot be NULL");
         return 0;
     }