Improve support for offline ESA services

When ESA services (SPICE bitbucket mainly) is not operational, the ESA metakernel resolution and kernel download raise timeout error because the tool will try to get the latest version of the metakernel available, eg:

TourConfig(mk='5.0b23.1')  # version='latest' by default

If the user provide an explicit SKD version, locally available (usually in ~/.planetary-coverage/esa-mk), the ESA API fetch will not append:

TourConfig(mk='5.0b23.1', version='v422_20230130_002')

This information should be reported into the docs.

Additionally, the URLError raised should be more explicit and it should be possible to list all the metakernel locally available (offline).