fix puterjs throwing errors after consuming message events not meant for it

[velzie]

puter.js uses message events to communicate between the Puter GUI and the app. However, not all message events that puterjs receives are sent from the GUI, they can be sent by the app itself, a subframe of the app, or even an external browser extension messaging with a content script (this is quite common). Other than these cases that I fixed, puter generally checks for the existence of msg and error before doing anything with the data, so it isn't a huge deal. The key msg is somewhat common though, so there's still a chance of it consuming messages that weren't sent from the GUI. There could probably be a namespaced key like puter_message so that this doesn't happen again in the future. I didn't implement this here because there's quite a few places where postmessage was used and there's no abstraction over it so it would get messy, but if that section is ever rewritten or revisited this should be kept in mind

[CLAassistant]

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