Import ZASM -> Crash

ZQuest will crash if when the user imports a ZASM script, they enter an invalid filename in the file lister.

https://youtu.be/h9wGq76O0vw

Note than I import ZASM for one of three main reasons:

1. When a quester provides a quest in which they need me to add an ffc, but they do not apply, or they do not have all of the ZScript files.
2. When recompiling requires rearranging headers, and importing precompiled ZASM is easier.
3. When I wish to 'idiot-proof' adding a script; usually ffcs.

This one is a bug. ffasm.cpp is due for a complete rewrite, but it's not a high priority for me right now (and ZASM will soon be a deprecated feature: still available for "hardcore scripters" but hidden by default from the ZQ GUI).

Fixed this today, in both 2.53 and 2.55. All of the ASM Import file dialogues now use the same file check error routine as importing a C-style script, and I corrected the error handling for them so that they don't have unhandled exceptions.