Skip core_handle, add a context manager to handle cubicweb errors
The context manager is also used to catch errors in render_view.
It handles the 'external' errors raised by cubicweb code.
The more internal errors, the one that should occur only in url resolving and
cubicweb controllers, are handled directly in CubicWebPyramidHandler.
ValidationError is handled by CubicWebPyramidHandler for now, but should
probably be handled by cw_to_pyramid
Related to #4291173
[pyramid\_cubicweb][] is one specific way of integrating [CubicWeb][] with a
[Pyramid][] web application.
### Features
* provides a default route that let a cubicweb instance handle the request.
### Usage
To use, install pyramid\_cubicweb in your python environment,
and then [include][] the package:
config.include('pyramid_cubicweb')
### Configuration
Requires the following [INI setting / environment variable][]:
* `cubicweb.instance` / `CUBICWEB_INSTANCE`
[pyramid\_cubicweb]: https://www.cubicweb.org/project/pyramid-cubicweb
[CubicWeb]: http://www.cubicweb.com/
[Pyramid]: http://pypi.python.org/pypi/pyramid
[include]: http://docs.pylonsproject.org/projects/pyramid/en/latest/api/config.html#pyramid.config.Configurator.include
[INI setting / environment variable]: http://docs.pylonsproject.org/projects/pyramid/en/latest/narr/environment.html#adding-a-custom-setting