Save errors only reported for the first image

[tst-jblair]

If there is an invalid file in the Imagery Sources list, it will only be reported as an error if it is the first file in the list.

1. In Fusion, open "Asset Manager" and select "New Imagery Resource."
2. Add an invalid file (e.g. "../download_tutorial.sh"), then add e.g. Blue Marble.
3. Save, and receive the expected error dialog.
4. Reverse the order of the files.
5. Save, and no error is reported.