* erepo: get rid of NoRemoteInExternalRepoError
This exception hasn't been making any sense at all since we've introduced
get and import, as they can be chained in repos.
* erepo: get rid of NoOutputInExternalRepoError
This is dead code these days.
* erepo: get rid of FileMissingError and PathMissingError
* api: preserve legacy exc wrappers
We used to have it named `cache` before, but these days it is clear that
you can have three odbs: just odb (e.g. in-memory), cache (typically local)
or remote (typically cloud). Fixing naming here before proceeding with
introducing `out.odb` and `out.cache`.
In preparation for migration to https://github.com/iterative/dvc-data/pull/304