### Problem
When a user clicks the merge button on a pull request and their quota is exceeded, the UI displays ( cf screenshot ):
> Network error SyntaxError: Unexpected token 'Q', "Quota exceeded." is not valid JSON
### Fix
Read the response body as text first with `resp.text()`, then attempt `JSON.parse()`. If parsing succeeds, use the existing `errorMessage` logic. If it fails, display the raw text directly in the error toast.
This is the same approach already used by Dropzone for attachment uploads, where the `error` event handler passes the response body directly to `showErrorToast`. ( cf screenshot )
### Tests for JavaScript changes
The function is not exported, I cannot create a unit test. Do you want me to export all the logic in an exported function ?
### Release notes
- [X] This change will be noticed by a Forgejo user or admin (feature, bug fix, performance, etc.). I suggest to include a release note for this change.
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/12635
Reviewed-by: Mathieu Fenniak <mfenniak@noreply.codeberg.org>
