Is it possible to make Preview prompt a dialog before quitting? I had multiple documents open in Preview, and then I accidentally pressed Cmd+Q, which quit the entire Preview program. Is there a way to make Preview prompt me asking if I want to close all documents before quitting?
There's a global setting, which all Apple apps respect [in fact there are two, either of which may help]

System Preferences > General >
Ask to keep changes when closing documents &amp;
Close windows when quitting an app



Checking the first will then ask about unsaved changes, though not about any document that is unedited.
Unchecking the second will make it so that simply re-launching Preview will restore all documents that were open at last Quit.