Fix path_shift fall back always triggering

[Voultapher]

Instead of always doing a negative root path shift,
even after a successful return, only catch exceptions.

[Voultapher]

@defnull the last time this was changed, is 7 years ago by you. I fail to understand why doing request.path_shift(-path_depth) even after a successful return could be the wanted behavior. Can you please shine some light on this.

[defnull]

If I remember correctly, because after_request handlers installed to the parent app should see the original state, not the modified one.

[Voultapher]

Ok, let me see if I can come up with a test for it.

[Voultapher]

The use case no longer matters for me so I'm closing this.