Shouldn't `get_manager_key` raised a `Missing_manager_contract` error instead of an `Unrevealed_manager_key` one?

The following discussion from !5182 (merged) should be addressed:

• @dianegalloiswong started a discussion: (+3 comments)

    Contract.must_be_allocated ctxt (Contract.Implicit source) >>=? fun () ->
    match revealed_key with

  As discussed with @ZaynahDargaye @ACoquereau and @vbotbol last week, adding this check doesn't change which operations are successfully prechecked, but it allows for a different error to be raised when the contract doesn't exist.

Following an offline discussion with the crew members (notbaly @ACoquereau and @dianegalloiswong), the question of whether it makes sense for get_manager_key to raise an Unrevealed_manager_key error and not a Missing_manager_contract one.

Looking at the coverage, the branches of that function are neither covered by !5182 (merged)'s master base, nor it's head.