mender-artifact modify only works if payload is an ext4 image

I would assume that "mender-artifact modify" does work as intended but the bug here in my opinion is that it does not state anywhere that only ext4 images are supported.

And the error messages do not really help (xfs payload in below examples),

Mender Artifact version: 2.3.0

mender-artifact modify artifact-signed.mender -k private.key
(fsck error: exit status 8)

Mender Artifact version: 2.4.0

mender-artifact modify signed.mender -k private.key 
Error selecting images for modification: error validating signature

Acceptance criteria:

• Improve error messages when unsupported images are used
• "mender-artifact modify --help" should cover the limitations of the command in e.g the DESCRIPTION section.
• Add warning in documentation that this command only works on ext4 images