handle non-aborting unwinds from recursive-edit

[Zoybean]

The function recursive-edit may unwind to its caller if any of the following forms is evaluated during the recursive edit:

• (abort-recursive-edit) causing recursive-edit to quit
• (throw 'exit t), causing recursive-edit to quit
• (throw 'exit STRING), causing recursive-edit to (error STRING)

The advice transient--recursive-edit behaves correctly in all of the above cases, except in the 3rd case if the error signal is caught after transient--recursive-edit unwinds and before the whole transient unwinds. In this case, the transient is left in a poor state. See #425 for more detail on reproducing this.

Adding an unwind-protect to the advice resolves this, but I am unsure if any of the contents of the cond ought to change. Specifically, the first arm of the cond appears to be trying to pre-empt the unwinding and tidy up, but prior to this PR, this arm does not run in an unwind anyway, and transients seem to handle unwinding from recursive-edit just fine. However, I am not confident to remove this - I do not know if there are any edge-cases that this code was added to handle.

Fixes #425

[Zoybean]

Alas, it looks like this PR fixes my case but breaks magit-commit. This might need further investigation.