VAULT-8719 Support data array for alias clash error response so UI/machines can understand error

[VioletHynes]

Note that I did need to do a little black magic here. The response as read by the logical framework is the same (nil response, non-nil error), but raw API requests give the full data array. This seems like a pretty good solution to me, as it doesn't return both a resp and an error to the logical framework, but I'm open to other thoughts and opinions and as to whether or not this is the right kind of black magic.

The approach is broadly extensible, in the sense that we can follow the same method again in the future, too.

The body of a response after this change will look a little like this (in the case of a double clash):

{
   "errors":[
      "3 errors occurred:\n\t* toEntity and at least one fromEntity have aliases with the same mount accessor, repeat the merge request specifying exactly one fromEntity, clashes: \n\t* mountAccessor: auth_github_41e995e1, toEntity ID: f41ba0e4-4492-04a5-e850-0715c39d600c, fromEntity ID: 12239930-f672-37ab-cd3d-668b79c51043, conflicting toEntity alias ID: a24f4921-27e6-e9dd-8a5a-e08ade178e49, conflicting fromEntity alias ID: 920efcfc-9406-a5c7-3061-283e3935fcc4\n\t* mountAccessor: auth_userpass_c13afd8e, toEntity ID: f41ba0e4-4492-04a5-e850-0715c39d600c, fromEntity ID: 79fcbd36-c92d-60ba-bd3e-af9836311eea, conflicting toEntity alias ID: 2d6617ee-4dd6-8445-c818-7b484e95b433, conflicting fromEntity alias ID: dc843aed-df0e-49d7-0ab7-e0b105d7b5db\n\n"
   ],
   "data":[
      {
         "alias":"bob-github",
         "entity":"bob-smith",
         "entity_id":"f41ba0e4-4492-04a5-e850-0715c39d600c",
         "mount":"github",
         "mount_path":"auth/github/"
      },
      {
         "alias":"clara",
         "entity":"clara-smith",
         "entity_id":"12239930-f672-37ab-cd3d-668b79c51043",
         "mount":"github",
         "mount_path":"auth/github/"
      },
      {
         "alias":"bob",
         "entity":"bob-smith",
         "entity_id":"f41ba0e4-4492-04a5-e850-0715c39d600c",
         "mount":"userpass",
         "mount_path":"auth/userpass/"
      },
      {
         "alias":"alice",
         "entity":"alice-smith",
         "entity_id":"79fcbd36-c92d-60ba-bd3e-af9836311eea",
         "mount":"userpass",
         "mount_path":"auth/userpass/"
      }
   ]
}