[CLIENT] Saving a mastery page with a duplicate name causes 'unspecified platform error'

If you attempt to name a mastery page with a name that matches an existing page, the client shows a cryptic error "unspecified platform error'. The case sensitivity does not matter.

Reproduction Steps:

1. Open the mastery page editor. This can be accessed from either the frontend or from champion select.
2. Attempt to save a mastery page with a name that is already in use.
3. You will receive a 'unspecified platform error' and no longer be able to change mastery pages.
4. If editing from champion select, you will enter the game with 0/0/0 masteries.

Expected result: The mastery page should save with a new name or provide a localized error message. Changing mastery pages should function as expected.

Frequency: 10/10 Reproduction Rate

Thanks