Image uploads to instances with image size limits fail

[mykdavies]

User report: https://lemmy.world/comment/1394283

Device info

• Liftoff version: 0.10.9

Describe the bug

We're getting a 413 Payload Too Large response which our implementation of the Pictrs API is choking on.

Steps to reproduce

1. Logon to Lemmy.ee.
2. Make a new post.
3. Try to add an image.
4. Catch the exception

Expected behavior

There's no error handling in the upload() function in pictrs.dart, so we need to add that. Not sure if the latest backend version allows sites to encode max upload sizes somewhere so we need to see if we can pick that up or just try some "small enough" size behind the scenes The backend does not support providing this information, so I've raised an enhancement request: LemmyNet/lemmy#3655

In the meantime, I'll look at just trying some "small enough" size behind the scenes and let the user know and decide if they want to try another approach.