Add an error message when you export a web build in Forward+

[gwenckatz]

Currently when you start a new project, it tells you that Forward+ is not web-compatible, but it still allows you to export a web build without any warnings or errors. Could we add an error message saying that Forward+ is not compatible with WebGL, or maybe just disable web builds in Forward+ (with an error saying to change to gl_compatibility)?

[AThousandShips]

It shouldn't export for web with Forward+, there's a project setting changing the specific renderer, but it's not clear that it doesn't use the renderer that's selected in the editor, but this true for mobile devices as well

A warning here would be annoying for anyone who is aware of this and doesn't mind, it's fully possible to develop games that use Forward+ for desktop, Mobile for mobile devices, and Compatibility for web, in I'd say it's even recommended to not create entirely separate projects for each version

So to be clear: The setting in the editor top right is not the renderer that will run on a specific device or target, in fact it doesn't override the project setting overrides for each platform, it's simply the mode used to display things and used for debugging, adding a warning would IMO create more confusion by making it seem that the editor mode has anything to do with exports (it doesn't), and would be annoying for anyone who is aware of this option

[AThousandShips]

Note that it's now not even possible to select this renderer in the settings, following godotengine/godot#108276

So this is now only possible with manually editing the file