If the `<hypothesis-sidebar>` element is moved around in the DOM, this can cause
the sidebar to reload. There may also be other causes of the sidebar reloading
(eg. process crash for out-of-process iframe?). If the sidebar loads a second
time, it will fail to connect to the host frame since `PortProvider` will try to
re-use the `MessageChannel` it has already allocated, but sending that channels
ports will fail since they have already been transferred.

Recovering from this scenario fully involves a lot of changes since all the
places that have a connection to the sidebar would need to support replacing
that channel. Also various state in host/guest frames (eg. currently loaded
annotations) will be out of sync and need resetting.

What this commit does is just to handle the situation more gracefully, by
logging a meaningful error in the console and, after a delay, showing an error
message in the sidebar telling the user to reload the page. This also avoids
spamming Sentry [1] with errors about a situation that is out of our control. We
get a lot of error reports about this each month, mainly from certain
high-traffic pages that embed the client, but so far no actual complaints from
users about Hypothesis not working. Therefore it doesn't yet seem valuable
enough to do all the work to recover from a sidebar frame reload automatically.

[1] https://hypothesis.sentry.io/issues/2975780063/