If the following matches no files, we currently print an error:
```
atlas_install_joboptions( share/*.py )
```

However, no error is printed for the following cases
```
atlas_install_joboptions( share/*.py share/*.txt)
atlas_install_joboptions( share/*.py share/foo.txt)
atlas_install_joboptions( share/foo.py share/foo.txt)
```
if at least one element in the list returns a valid file.

Improve the check by requiring that each element in the list is a
valid entry.