fix: Check image can be read before opening

[dtrudg]

Description of the Pull Request (PR):

If an image cannot be read, attempts to open it with the various image format handling code will fail. Since we are using a fall-through approach to find the correct image format, these failures did not give a sensible error message.

Add an explicit check that the image is readable.

$ ls -lah overlay.img 
----------. 1 dtrudg dtrudg 1.0G May  4 10:55 overlay.img

Before...

$ singularity run --overlay overlay.img busybox_latest.sif 
FATAL:   while loading overlay images: failed to open overlay image overlay.img: image format not recognized

After...

$ singularity run --overlay overlay.img busybox_latest.sif 
FATAL:   while loading overlay images: failed to open overlay image overlay.img: /home/dtrudg/overlay.img is not readable by the current user, check permissions

This fixes or addresses the following GitHub issues:

• Fixes Confusing 'overlay image format not recognized' error when file is not readable (perms) #786

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR, linted with make check and tested this PR locally with a make test, and make testall if possible (see CONTRIBUTING.md).
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File