[MERGE] Improve error handling in msgeditor._run_editor.

[John Arbash Meinel]

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Andrew Bennetts wrote:
> A user on IRC just reported trouble using “bzr commit” without -m.  It
> turned out that somehow they had this line in their bazaar.conf:
> 
>     editor = ""
> 
> There are two issues here.  First is that executing "" causes EACCES, and
> _run_editor wasn't catching that.  The second is that the error reporting
> was very unhelpful in this case; it didn't report which command bzr was
> trying to run, or why.
> 
> So this patch fixes both.  It catches EACCES in _run_editor (which could
> happen if the editor isn't executable or readable as well), and it also
> emits a warning to the user if a configured editor is not exectuable, on the
> basis that that means their configuration/environment is probably not sane.
> It *doesn't* emit that warning when trying the hardcoded guesses
> (notepad.exe, /usr/bin/editor, etc), only for user-controllable values.
> 
> I'm interested in feedback about emitting a warning in this situation.  On
> one hand, it would have made this problem easier to diagnose — the user on
> IRC today probably wouldn't have needed to come to #bzr for help if we
> emitted it.  But maybe it's too chatty to complain if a user's environment
> is slightly broken?  I think probably this change is fine (it's usually not
> hard to set $EDITOR correctly), but I know extra output can irritate people,
> so let me know.
> 
> -Andrew.

BB:approve

Well, at least the updated version.

John
=:->

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