UnsupportedDataTypeConfigException doesn't say which config file the problem is

Makes sense to me. I updated the issue summary to include a beta phase evaluation.

Improving our exception, that is great!!

I like this patch.

We can test that the file is added to the message too. Just add an assertion in ConfigCRUDTest where the exception is tested.

Tests for #11.

+++ b/core/modules/config/src/Tests/Storage/FileStorageTest.php
@@ -76,4 +77,18 @@ public function testlistAll() {
+    } catch (UnsupportedDataTypeConfigException $e) {

catch must not be on the same line as the preceding closing curly bracket. Instead it must be on a new line.

Apart from this coding style nitpick, the patch looks RTBC. I confirmed the tests cover the new behavior, and that no existing behavior has changed.

Changes as per #14.

+++ b/core/lib/Drupal/Core/Config/FileStorage.php
@@ -95,12 +96,17 @@ public function read($name) {
+      throw new UnsupportedDataTypeConfigException(SafeMarkup::format('Invalid data type in config @name, found in file @file: @message', array(
+        '@name' => $name,
+        '@file' => $filepath,
+        '@message' => $e->getMessage(),
+      )));

SafeMarkup::format() is deprecated and should not be used in exceptions anyway. sprintf() or just plain old string concatenation should be used. Using an escaping method in an exception message can cause double escaping as exception messages are only escaped at the time of output.

Changes as per #16.

Nice! This is a really nice improvement

Committed/pushed to 8.1.x and cherry-picked to 8.0.x. Thanks!