[Bug]: M4B conversion does not check if it has permissions to move/create files before proceeding

[C0rn3j]

What happened?

I have set my library to be RO, and the M4B conversion tries to move data from and to my directory after the very-long conversion process.

What did you expect to happen?

A check that the directories are actually writable.

Steps to reproduce the issue

1. Put mp3's in a RO directory and try to convert them to M4B

Audiobookshelf version

2.11.0

How are you running audiobookshelf?

Debian/PPA

What OS is your Audiobookshelf server hosted from?

Linux

If the issue is being seen in the UI, what browsers are you seeing the problem on?

None

Logs

No response

Additional Notes

Mildly related to #3073, as by resolving this issue, the conversion would still fail, just earlier.