Erroneous error when selecting a username

[hibes]

Describe the bug
This username is already taken is displayed when an uppercase username value is rejected by matrix/synapse.

To Reproduce
Steps to reproduce the behavior:

1. Set up an external IdP with registration enabled
2. Register a new account using the IdP
3. When returning to matrix authentication service to select a username, try a new unique username containing an uppercase letter
4. Observe the erroneous error message This username is already taken
5. Remove the uppercase letter
6. Observe the expected behavior of registering the new username with matrix

Expected behavior
The error message should reflect the error from synapse (something like User ID can only contain characters a-z, 0-9, or '=_-./+' -- see below)

synapse-1  | 2025-03-08 20:19:57,259 - synapse.http.server - 130 - INFO - GET-2294 - <XForwardedForRequest at 0x7f0b3d7a83b0 method='GET' uri='/_synapse/admin/v1/username_available?username=Onecapitalletterisenough' clientproto='HTTP/1.1' site='8008'> SynapseError: 400 - User ID can only contain characters a-z, 0-9, or '=_-./+'

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Linux
• Browser: Firefox
• Version: 136

Smartphone (please complete the following information):
N/A

Additional context
N/A

[sandhose]

Thanks for the report! This is because we basically showed the first error, instead of checking the whole form correctly. I've opened #4173 to fix this