There should be runtime error when using EditorPlugin in runtime

[kubecz3k]

Godot version:
3.0.3

Issue description:
Since EditorPlugin can't be used in non-tools builds, there should be a runtime error in the editor when running a game using it. Not having this error can lead to potentially hard to debug problems on user ends when releasing the game (coming from #19649)

Steps to reproduce:

1. Create main scene of the project with a script
2. Add such code in script ready function:

editorPlugin = EditorPlugin.new();
add_child(editorPlugin);

3. Run the scene, observe that there is no indication that something is not right

[Zylann]

Note: it sounds like we could do a check for any editor-specific classes, however that's not always possible in case the plugin comes with debugging capabilities (which would require access to editor-only classes into a running game). In Unity, what is done is that the exporter has an option to strip out such code (Debug.Log, Profiler calls etc)

[Calinou]

Running the snippet gives the following error messages in Godot 3.2.1, so I'll consider this resolved.