Write up comprehensive advice to "Tor unexpectedly exited", and link to it from inside Tor Browser

We've been getting a steady stream of people who fail to run Tor Browser, who get the "Tor unexpectedly exited" error and they have no ideas what to do.

The internets are full of weird advices about what to do in this situation back in 2014, few of which apply now, so people hunting in google are unlikely to solve this on their own.

I think the advice differs depending on the OS -- on Windows, the first step is about disabling antivirus and firewall; on OS X, the advice is...about finding the old tor process and killing it? Or something else?

We should write up a comprehensive "what the error means, what to do, in what order" explanation, and stick it on support.torproject.org so people can find it and so we can point people to it. And then we should link to it from inside Tor Browser, when the thing goes wrong, so there is a canonical place for users to read what they should do.

(And longer term, we should improve the automated handling of this error inside Tor Browser, but that can come later.)