Do early return so we don't try to generate a thumbnail when we cannot.

[rtibbles]

Summary

Description of the change(s) you made

• We do a check to see if we have the information we need to generate a thumbnail, but then try to generate the thumbnail anyway
• This fixes that by returning early

Manual verification steps performed

1. Upload the epub in the archive here: https://github.com/learningequality/studio/files/9887092/TestGoogleEPUB.zip
2. Try to generate a thumbnail.
3. Confirm it tells the user there is an error, but no console errors are shown.

Fixes #3770

[marcellamaki]

Change makes sense and manual testing working as expected

[pcenov]

I confirm, no errors in the console when a thumbnail is not being generated now for that type of EPUBs.