Add a sanitizeError option and specs

[Stuk]

⚠️ First pass at this idea. Happy to have a discussion about it. Also, the readme should be updated, but I didn't want to put that effort in until the approach has been approved.

Error objects often contain information, such as the stack, that should not be exposed to the client.

This adds the ability to give a sanitizeError function to the Connection that will be called whenever an Error is encoded. The return value of this function is used in place of the original error.

[mczepiel]

I, for one, would appreciate having such an ability to prevent leaking server information. Looks straightforward enough to serve my needs.

[kriskowal]

I think that it might be sufficient to sanitize errors by default, and turn off sanitization with a debug option. Perhaps "debug" for one and "trace" for logging.

[mczepiel]

@Stuk had a nice idea that benefitted from being able to alter how the sanitization happens: reporting some id for the exceptional event to the user for support purposes. (Though that sort of thing could be implemented without informing the end user, with the benefit of it appearing all the more magical)