[MERGE][#246233] Allow ConnectionReset to propagate from read_mergeable_from_url.

[Andrew Bennetts]

This small change fixes one of the remaining causes of TooManyConcurrentRequests
errors.  "bzr pull" and "bzr merge" both first try opening the URL as a bundle
via read_mergeable_from_url, but if the connection failed that function was
raising NotABundle instead of propagating the ConnectionReset.

This fix is a two-liner (catch ConnectionReset explicitly and re-raise it), and
I've included a test which unfortunately is a bit longer, but still quite
simple.

Perhaps a deeper fix would be to re-examine our exception hierarchy;
maybe ConnectionReset should not subclass TransportError.  It's also not
immediately obvious why read_mergeable_from_url catches TransportError — I'm
guessing that's what “transport.get('directory-name')” usually raises?  A
comment is probably warranted.  But instead of letting the perfect be the enemy
of the good, I'm submitting a simple and clearly correct patch.

With this patch, users get a fairly sensible error message rather than a
traceback, e.g.:

$ ./bzr pull bzr+ssh://no-such-user@bazaar.launchpad.net/
No such Launchpad account: no-such-user
Permission denied (publickey).
bzr: ERROR: Connection closed: please check connectivity and permissions (and try -Dhpss if further diagnosis is required)

-Andrew.

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