fix: descriptive upload error messages

[SychO9]

Fixes #141

Changes proposed in this pull request: Handles the InvalidUploadException for end user consumption rather than a non-understandable 500.

Confirmed

• [ ] Frontend changes: tested on a local Flarum installation.
• [ ] Backend changes: tests are green (run composer test).

Required changes:

• [ ] Related Flarum core extension PR's: (Omit this section if irrelevant)