Greg Spencer 1633c47dda
Improve asserts on Element.mount (#153477)
## Description

In debugging an issue with mount, I hit one of these asserts, and I thought it would be a much better assert with some context.

## Tests
 - No test changes because the change is only to the output string of an assert.
2024-08-15 21:30:03 +00:00
..
2024-08-15 16:39:28 +00:00