dnf: add more specific error message for GPG check (#76194)

Fixes #76192

changelogs/fragments/76192-dnf-specific-gpg-error.yml

@@ -0,0 +1,2 @@
+minor_changes:
+  - dnf - add more specific error message for GPG validation (https://github.com/ansible/ansible/issues/76192)

lib/ansible/modules/dnf.py

@@ -1298,7 +1298,7 @@ class DnfModule(YumDnf):
                             fail = True
 
                         if fail:
-                            msg = 'Failed to validate GPG signature for {0}'.format(package)
+                            msg = 'Failed to validate GPG signature for {0}: {1}'.format(package, gpgerr)
                             self.module.fail_json(msg)
 
                 if self.download_only:

test/integration/targets/dnf/tasks/gpg.yml

@@ -57,6 +57,7 @@
         that:
           - res is failed
           - "'Failed to validate GPG signature' in res.msg"
+          - "'is not signed' in res.msg"
 
   always:
     - name: Remove rpm-sign (and test package if it got installed)