region export permission

[kswang1029]

Describe the bug
This is reported by a user via email.

I think when saving crtf regions, if you don't have write permission,
carta still gives you the green popup that says it has saved the crtf
regions for file xxx, but doesn't save anything. It should probably
not claim to have saved the region in this case.

[test: make a crtf file, make it non-writeable, then try to save a
region from carta to this file - it will first ask you whether you
want to overwrite the file, you say yes, then you get the green popup
suggesting it has saved, but in fact it has not]

To Reproduce
Steps to reproduce the behavior:

[test: make a crtf file, make it non-writeable, then try to save a
region from carta to this file - it will first ask you whether you
want to overwrite the file, you say yes, then you get the green popup
suggesting it has saved, but in fact it has not]

Expected behavior
If there is no permission to save a region file, the returned message should be a failure.

Platform info (please complete the following information):
the user did not provide platform info

• OS [e.g. macOS Monterey]:
• Browser [e.g. chrome, safari, electron app]:
• Browser version [e.g. 22]:
• Backend branch [e.g. dev, v3b2 release]:
• Frontend branch [e.g. dev, v3b2 release]:

[veggiesaurus]

@pford can you handle this for the stable release? It should be relatively easy to fix, I imagine.