Errors: Make errors the same in dev as prod (#77366)

When running in dev mode, error messages would contain an additional "error" property alongside "message". Since this causes confusion, that has been removed and now error messages are the same both modes (using "message").

pkg/api/admin_users_test.go

@@ -230,7 +230,7 @@ func TestAdminAPIEndpoint(t *testing.T) {
 
 			respJSON, err := simplejson.NewJson(sc.resp.Body.Bytes())
 			require.NoError(t, err)
-			assert.Equal(t, "user already exists", respJSON.Get("error").MustString())
+			assert.Equal(t, "User with email '' or username 'existing@example.com' already exists", respJSON.Get("message").MustString())
 		})
 	})
 }