Gitaly is in the process of migrating UserSquash to use detailed errors
like we already do for UserMergeBranch and UserSquash. This change will
break the current calling convention, where Gitaly will return some
errors by returning successfully, but with an `git_error` field set in
the response. Instead, Gitaly will return a proper error for all
conditions but give clients the ability to distinguish error cases via
error details.

Prepare UserRebaseConfirmable for this change by handling those new error
details. Theoretically, this allows us to implement more fine-grained
handling of these errors now: `AccessCheck` contain an error from
Gitaly calling the Gitlab API, and `MergeConflictErrors` contain
information about the files that have been conflicting. To ease the
transition though we're treating those errors exactly the same as we did
previously, where we raised a `PreReceiveError` when there was a error
during pre_receive and `GitError` otherwise. We can still iterate on this
at a later point.

The end result should be that there is no real user-visible change in
how we handle UserRebaseConfirmable.