Match specific errors on scrobble_rules duplicate

Pin the duplicate-rule error to the match_value field and "has already been
taken" message, matching the unique_constraint error_key in the changeset.

Refs #176

test/music_library/scrobble_rules_test.exs

@@ -72,7 +72,8 @@ defmodule MusicLibrary.ScrobbleRulesTest do
 
       assert scrobble_rule.type == :album
 
-      assert {:error, _changeset} = ScrobbleRules.create_scrobble_rule(valid_attrs)
+      assert {:error, changeset} = ScrobbleRules.create_scrobble_rule(valid_attrs)
+      assert %{match_value: ["has already been taken"]} = errors_on(changeset)
     end
 
     test "create_scrobble_rule/1 with invalid data returns error changeset" do