What the user expects when error happens?

[bartbes]

But then, if the app is using the library and in some point the app requires the user to insert "foobar.png", and he inserts "foobar.pngg" by mistake, and I throw and error(), the user may lost a lot of work. Let's say someone uses my library in a Photoshop-like program. If the app simply crashes, he me lost hours of work.

That is up to the creator of the application, if this is well documented, and this can occur, that creator should just use pcall and handle it, or, like kikito said, check beforehand whether the input is going to be valid.