allow the commit_veto to handle squashed exceptions

[mmerickel]

Basically this allows the commit_veto to decide to commit a transaction even if the request contains a squashed exception.

This change is bw-incompatible for anyone using a commit_veto as the veto will now be consulted for some requests that previously (all the way back to pyramid_tm 1.x) were simply aborted.

I think this is a nice balance - pyramid_tm still has no hook in which you can specify that you want to commit an unhandled exception but I feel like that's alright. The advantage of this particular PR is that HTTPFound etc are currently squashed by Pyramid and that fact combined with this change is that the veto hook could allow those to be committed. I'd even consider making the default_commit_veto the default for pyramid_tm and pushing a 3.0. I'd only change the veto if it was configured to abort on 4xx/5xx and request.exception was not None (currently it aborts without checking request.exception.

fixes #64

[mmerickel]

I decided to just ship this as-is and not worry about changing the commit veto. Anyone can do that if they choose.