Error if module was not set up.

[stelcheck]

Ref: https://github.com/mage/mage-sdk-unity/blob/develop/Mage/Module.cs#L86

Description

When attempting to make a call to a module which was not set up, you get a null error.

Desired behavior

Modules are singletons; they should have a protected field that tracks if the module was initialized, and calls should error out early if:

1. The module has not been initialized
2. If we attempt to execute a user command that is not defined in the module

cc @nullorvoid @AlmirKadric