ClientRpc and ServerRpc decorators should give an error if attached to a non-NetworkBehaviour class method #1911
Description
Is your feature request related to a problem? Please describe.
I'm finding it easy to accidentally add [ClientRpc] or [ServerRpc] methods to MonoBehaviour-derived classes, where clearly they won't do anything. This leads to a bunch of confusion while I try to figure out what's wrong with the networking only to do the forehead-slap when I notice that the class hasn't been switched to NetworkBehaviour.
Describe the solution you'd like
At a minimum, a runtime error with a descriptive error message would be helpful. Something like "FooClientRpc was added to a class that doesn't derive from NetworkBehaviour - as a result, FooClientRpc will never be called."
Better would be a similar error logged when in the editor. Compile-time errors would be ideal if possible, of course.
Describe alternatives you've considered
Beyond "vigilance" (which can be assumed doesn't work), if it becomes a more serious issue then I'd have to rig some sort of linting rule to catch this.