linking to the openapi spec is enough

configuration removed since 4ecb05ca7932

Mount on "/" when we are on an older version of cubicweb missing the receives-base-url-path OR when the receivese-base-url-path option is set to False. This option has been added in version 4.2.

Otherwise we are serving our routes on the full base url.

Closes #34.

Closes #702.

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