Improve loaders to deal with new visit status events

The model and db now supports the not_found and failed status. Update the loader implementations to reference such information.

Note: It's also the opportunity to make the loader reference the visit-status type when discussing with the storage [1]

This is a state diagram explaining the different statuses :

• [1] !170 (closed)

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Migrated from T3030 (view on Phabricator)

mentioned in merge request !170 (closed)

mentioned in merge request !171 (closed)

mentioned in merge request !172 (closed)

mentioned in merge request swh-scheduler!218 (closed)

mentioned in merge request !173 (closed)

mentioned in merge request !174 (closed)

mentioned in merge request swh-loader-git!58 (closed)

mentioned in merge request swh-loader-git!59 (closed)

mentioned in merge request swh-loader-svn!49 (closed)

mentioned in merge request swh-loader-svn!50 (closed)

mentioned in merge request swh-loader-mercurial!44 (closed)

mentioned in merge request swh-loader-mercurial!45 (closed)

mentioned in merge request swh-web!489 (closed)

added Core Loader priority:Normal labels

mentioned in commit 65a51357

mentioned in commit 4500063a

mentioned in commit fb7ee433

changed the description

mentioned in commit swh-scheduler@aa507ac5

changed the description

changed title from Improve loader to deal with new visit status events to Improve loaders to deal with new visit status events

mentioned in commit 96b949cf

mentioned in commit 945ff242

changed the description

mentioned in commit swh-loader-svn@71e3b447

mentioned in commit swh-loader-svn@ddc18d08

mentioned in commit swh-loader-git@11074801

mentioned in commit swh-loader-git@c2cd5fed

mentioned in commit swh-loader-mercurial@1f96493e

mentioned in commit swh-loader-mercurial@4c84a86b

mentioned in commit swh-web@a38de155