Backend crash if file attached to is removed

[kgolap]

Describe the bug
If the image being viewed gets removed from disk the back end crash

This is what the backend messages before crashing

terminate called after throwing an instance of 'casacore::AipsError'
what(): File::getstat error on /lustre/kgolap/BUGS/CAS-13814/hpg_v100_basic_cube.pb: No such file or directory
/tmp/.mount_cartaknIhjk/AppRun: line 26: 104879 Aborted (core dumped) $DIR/bin/carta_backend "$@"

To Reproduce
Steps to reproduce the behavior:

1. View an image in the frontend
2. delete the image on disk while the frontend is displaying it

Expected behavior
After receiving the signal that the image is no longer there it should just remove the image from the display (i.e
forcefully close it) rather than the backend quitting

Platform info (please complete the following information):

• OS: Linux RHEL 7
• Browser Firefox
• Version 102.10 esr

[kswang1029]

@veggiesaurus @confluence @YuHsuan-Hwang I guess in this case, we need a mechanism to let backend trigger. CLOSE_FILE. Currently, the close file action is only triggered by the frontend (ie user).