CLI: use openDocumentSimple multiple times instead of openDocumentsSimple

openDocumentsSimple was apparently created for a purpose different from
just opening multiple documents in succession. It has some slightly
strange behaviour when used like this (e.g. you can open the same document
twice).
Revert "If code without debug symbols or file doesn't exist raise the"

This reverts commit 2c33ab16bf88ce2893b95c72320a7b1caca6f4e2.

That commit brings more bad than good. As DisassembleWidget appers
whenever you attach to a program or edit breakpoints, which can
become very, very, very annoying.