Error when attempting to upload webp as cover

### Problem

OL lets you upload webp images, but then fails with "Unable to render this page."


This appears to be a separate issue from #7250

### Reproducing the bug

1. On an edition, select 'Manage Covers'
2. Select 'Choose File'
3. From the system image picker, choose a webp image

* Expected behaviour:
OL either supports upload of webp images, or prevents uploading them in the first place

* Actual behaviour:
The generic error "Unable to render this page." is displayed in the cover management modal.
The main page also displays the error message `/openlibrary/openlibrary/templates/covers/add.html: error in processing template: AttributeError: 'code' (falling back to default template)`.

### Context

- Browser (Chrome, Safari, Firefox, etc): Safari
- OS (Windows, Mac, etc): iPadOS 18.2
- Logged in (Y/N): Y
- Environment (prod, dev, local): prod

### Breakdown

#### Requirements Checklist
* [ ]

#### Related files

*

#### Stakeholders

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