screenshot tool fails to save

[qubodup]

What happens
1. I press "print" to make a screenshot and save it to a plugged in vfat usb drive
2. the file name by default contains colons ":"
3. Failed to open '/path/to/image:file.png' for writing: Invalid argument
4. I press OK, the dialog closes, the screenshot is lost.

What should happen:
3. I get a message that explains the reason why writing is impossible (out of space? illegal characters?)
4. I press OK and get to pick a new name/location, the screenshot is not lost.

Easiest solution would be to not have ":" in the default generated names.