pathpolicy: tweak error messages and add test

[mvo5]

Trivial tweak for the errors returned from PathPolicies.Check() so that they are a bit more user friendly.

See also osbuild/bootc-image-builder#601

[achilleas-k]

These messages never reach the user. We run through them when checking the blueprint and list the ones that failed to validate without giving a specific:

images/pkg/blueprint/filesystem_customizations.go

Lines 74 to 89 in 98860d0

	// CheckMountpointsPolicy checks if the mountpoints are allowed by the policy
	func CheckMountpointsPolicy(mountpoints []FilesystemCustomization, mountpointAllowList *pathpolicy.PathPolicies) error {
	invalidMountpoints := []string{}
	for _, m := range mountpoints {
	err := mountpointAllowList.Check(m.Mountpoint)
	if err != nil {
	invalidMountpoints = append(invalidMountpoints, m.Mountpoint)
	}
	}
	if len(invalidMountpoints) > 0 {
	return fmt.Errorf("The following custom mountpoints are not supported %+q", invalidMountpoints)
	}
	return nil
	}

We should also make a similar change to what you proposed on osbuild/bootc-image-builder#601 (comment)

[mvo5]

These messages never reach the user. We run through them when checking the blueprint and list the ones that failed to validate without giving a specific:
[..]
We should also make a similar change to what you proposed on osbuild/bootc-image-builder#601 (comment)

Thank you! Nice catch! I updated this PR to also include the improved error.

[achilleas-k]

Wonderful!

[ondrejbudai]

Yay!