Add feature available/implemented errors

This adds two new error codes: GLFW_FEATURE_UNAVAILABLE for when
a GLFW feature cannot be reasonably implemented on that platform, and
GLFW_FEATURE_UNIMPLEMENTED for when it can be but has not been yet.

This replaces the current situation where the Wayland code emitted
GLFW_PLATFORM_ERROR in both cases while the macOS code silently did
nothing.

If your application exits on any GLFW error, these error codes should at
least be easy to filter out from that behavior.

Ideally, GLFW_FEATURE_UNAVAILABLE should be rare and
GLFW_FEATURE_UNIMPLEMENTED should never be emitted at all.

Fixes #1692.

src/cocoa_window.m

@@ -982,7 +982,8 @@ void _glfwPlatformSetWindowTitle(_GLFWwindow* window, const char* title)
 void _glfwPlatformSetWindowIcon(_GLFWwindow* window,
                                 int count, const GLFWimage* images)
 {
-    // Regular windows do not have icons
+    _glfwInputError(GLFW_FEATURE_UNAVAILABLE,
+                    "Cocoa: Regular windows do not have icons on macOS");
 }
 
 void _glfwPlatformGetWindowPos(_GLFWwindow* window, int* xpos, int* ypos)
@@ -1376,6 +1377,8 @@ void _glfwPlatformSetWindowOpacity(_GLFWwindow* window, float opacity)
 
 void _glfwPlatformSetRawMouseMotion(_GLFWwindow *window, GLFWbool enabled)
 {
+    _glfwInputError(GLFW_FEATURE_UNIMPLEMENTED,
+                    "Cocoa: Raw mouse motion not yet implemented");
 }
 
 GLFWbool _glfwPlatformRawMouseMotionSupported(void)