Draft: Bug 41385: BootstrapError is never notified

Made sure that BootstrapError is notified.

Also, cleaned a little bit TorBootstrapRequest, and moved some common code to an internal method.

Then, I removed the TorProcessDidNotStart: it was used only for the old launcher, but it is not used for about:torconnect.

However, while doing so, I have noticed that we could improve the error reporting in a few ways. For example, modified TorMonitorService to keep track of startup errors, even though we don't show at the moment.

@richard asking review from you, since you already know the mechanisms of this part of the browser, so that you can tell me what you think of this idea, and how we could follow up in about:torconnect/about:tor (e.g., improve the red screen of death). (I have already opened #41389 for adding the latest log line in about:torconnect).

Partly overlaps with !412 (merged).

Closes #41385 (closed).