Pleroma.User.Backup: fix some dialyzer errors

lib/pleroma/user/backup.ex:207:call
The function call will not succeed.

:zip.create(
  string(),
  [:"\"actor.json\"", :"\"outbox.json\"", :"\"likes.json\"", :"\"bookmarks.json\""],
  [{:cwd, binary()}, ...]
)

will never return since the success typing is:
(
  atom() | [atom() | [any()] | char()],
  [
    atom()
    | [atom() | [any()] | char()]
    | {atom() | [atom() | [any()] | char()], binary()}
    | {atom() | [atom() | [any()] | char()], binary(),
       {:file_info, :undefined | non_neg_integer(),
        :device | :directory | :other | :regular | :symlink | :undefined,
        :none | :read | :read_write | :undefined | :write,
        :undefined | non_neg_integer() | {_, _}, :undefined | non_neg_integer() | {_, _},
        :undefined | non_neg_integer() | {_, _}, :undefined | non_neg_integer(),
        :undefined | non_neg_integer(), :undefined | non_neg_integer(),
        :undefined | non_neg_integer(), :undefined | non_neg_integer(),
        :undefined | non_neg_integer(), :undefined | non_neg_integer()}}
  ],
  [
    :cooked
    | :memory
    | :verbose
    | {:comment, string()}
    | {:compress, :all | [[any()]] | {:add, [any()]} | {:del, [any()]}}
    | {:cwd, string()}
    | {:uncompress, :all | [[any()]] | {:add, [any()]} | {:del, [any()]}}
  ]
) ::
  {:error, _}
  | {:ok,
     atom() | [atom() | [any()] | char()] | {atom() | [atom() | [any()] | char()], binary()}}

and the contract is
(name, fileList, options) :: retValue
when name: :file.name(),
     fileList: [:FileSpec],
     fileSpec:
       :file.name() | {:file.name(), binary()} | {:file.name(), binary(), :file.file_info()},
     options: [:Option],
     option: create_option(),
     retValue:
       {:ok, FileName :: filename()}
       | {:ok, {FileName :: filename(), binary()}}
       | {:error, Reason :: term()}

lib/pleroma/user/backup.ex

@@ -22,6 +22,8 @@ defmodule Pleroma.User.Backup do
   alias Pleroma.Web.ActivityPub.UserView
   alias Pleroma.Workers.BackupWorker
 
+  @type t :: %__MODULE__{}
+
   schema "backups" do
     field(:content_type, :string)
     field(:file_name, :string)
@@ -195,6 +197,7 @@ defp wait_backup(backup, current_processed, task) do
   end
 
   @files ['actor.json', 'outbox.json', 'likes.json', 'bookmarks.json']
+  @spec export(Pleroma.User.Backup.t(), pid()) :: {:ok, String.t()} | :error
   def export(%__MODULE__{} = backup, caller_pid) do
     backup = Repo.preload(backup, :user)
     dir = backup_tempdir(backup)
@@ -204,9 +207,11 @@ def export(%__MODULE__{} = backup, caller_pid) do
          :ok <- statuses(dir, backup.user, caller_pid),
          :ok <- likes(dir, backup.user, caller_pid),
          :ok <- bookmarks(dir, backup.user, caller_pid),
-         {:ok, zip_path} <- :zip.create(String.to_charlist(dir <> ".zip"), @files, cwd: dir),
+         {:ok, zip_path} <- :zip.create(backup.file_name, @files, cwd: dir),
          {:ok, _} <- File.rm_rf(dir) do
-      {:ok, to_string(zip_path)}
+      {:ok, zip_path}
+    else
+      _ -> :error
     end
   end
 
@@ -382,6 +387,8 @@ def do_process(backup, current_pid) do
         [:file_size, :processed, :state]
       )
       |> Repo.update()
+    else
+      e -> {:error, e}
     end
   end
 end