Give user better feedback than "NotBranchError", and suggestion for recovery.

If a hosted branch could not be mirrored, the branch page will display an error supposedly explaining why. For hosted branches, this is almost always a "Not a branch". However, as described in https://lists.ubuntu.com/mailman/private/launchpad/2007-February/013833.html there are multiple causes of the "Not a branch" error that users see.

We should automate the diagnosis of these causes, and suggest an appropriate remedy to the user. If no recognised cause is determined, obviously reporting a the original NotBranchError is an acceptable fallback.

For example, we could detect and report "$LOCATION is an empty directory. Remedy: Using bzr 0.15 or newer, do 'bzr push --use-existing-dir $LOCATION'."

("--use-existing-dir" is a new option in the upcoming bzr 0.15 release)

Another example: "$LOCATION contains a repository (.bzr/repository) but no branch (.bzr/branch). Remedy: Using bzr 0.15 or newer, do 'bzr push $LOCATION' again."

(again, I specify a bzr version because bzr 0.15 will be better at resuming interrupted pushes than earlier versions)

Both of these diagnoses are fairly easy to automate, and the other commonly occuring problems probably are too.

Related to bug #48064. Bug #86420 is also related.