Show why a review failed to open instead of spinning forever

[derjabkin]

Show why a review failed to open instead of spinning forever

Problem

Opening a pull request or a branch from the start page puts up the "Preparing the
review workspace" overlay. If the open failed - gh not reachable, a fetch refused,
a ref that does not resolve - the exception went to the global handler, which writes
one log line. Nothing took the overlay down, so the window stayed behind a spinner
that would never stop, with no word of what went wrong.

Change

• A failed open now replaces the checklist in the overlay with the reason (the first
  line of the tool's own error for a failed command) and a Close button. The same text
  goes to the start page's status line and the Log pane.
• If "Continue now" was pressed before the failure arrived, the overlay comes back to
  show it.
• An open cancelled by a later open leaves the later one's overlay alone.

Cancelling the open also has to mean the review already on screen is untouched:

• OpenPrAsync cleared the offline flags, and OpenLocalRangeAsync overwrote
  DirtyWorktreePath, before anything had been fetched. Both now assign state only
  once the diff has been read, so a failure up to that point changes nothing.

Screenshot

Not covered

Opens that do not go through the start page - the PR list pane, reload, the auto-open
with no start page - never showed the overlay and report failures as before.

Testing

Builds with 0 warnings and 0 errors. The error state has not been exercised in the
running app. To check it: open a pull request that has no cached snapshot with the
network off.