Only log exception when it leads to a 500 error.

Otherwise exceptions raised on commit are not caught by the api exception view deriver.

refactor(exceptions): define custom api exception, a view deriver to raise them and view exception to display them

Fixes #32 and cubicweb/cubicweb#680

Closes #30.

Closes #30.

Which is more pythonic.

The possibility to register all routes within a route prefix allow to remove
this complex code.

Also remove unecessary check on openapi 'paths' property presence.

The base url is already here for this.