Batch export: swap is-file and missing-directory cases to intended use

I found some unexpected behaviour in batch export: #5443 (closed)

Namely, a missing directory in the export path causes the error message for a directory name that is an existing file. Conversely, an existing file of the same name causes the dialog to create a missing directory.

I think this is fixed with inverting a single if-statement, as in this commit. I tested it by applying the change onto 1.4.x. Sadly I wasn't able to test it with this exact branch due to my old OS.

There are some things I don't know:

• whether it should lead to the dialog box with a progress bar and a "cancel export" button. In this situation, cancelling the export seems like the only option.
• whether a test case should cover this.
• whether it needs to be tested on both the 1.4.x and master branches.

I hope this is useful and look forward to your comments. Thank you!