Related to #416, but don't explain it.
This PR just handle a system image as invalid instead of crashing when :

• File is not a zip file (so like Unable to select LMODroid #416 )
• File is a zip file but does not contain metadata file

For now, only IMG-e-1.18-t-20231207360613-stable-FP5.zip is known to be in the second case, and it's a very strange installation package, OAI cannot handle that for now (related to #288 ?), so the file is shown as invalid.
Maybe we can explain it with more details to the user?