Spec: throw error when run() or selectURL() is called during initial module script execution

[alexmturner]

In the definitions of run() and selectURL(), it says "If addModule() has not yet been called, [error]". To align with the implementation, this should probably be modified to something like "If addModule()'s returned promise has not yet resolved"? (i.e. we need to wait for the module script execution to complete first)