avocado.utils.process: Unify CmdError messages

The interrupted message lacked '' around the cmd and ignored the
additional_text.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/process.py

@@ -83,8 +83,9 @@ class CmdError(Exception):
     def __str__(self):
         if self.result is not None:
             if self.result.interrupted:
-                return "Command %s interrupted by user (Ctrl+C)" % self.command
-            if self.result.exit_status is None:
+                msg = "Command '%s' interrupted by user (Ctrl+C)"
+                msg %= self.command
+            elif self.result.exit_status is None:
                 msg = "Command '%s' failed and is not responding to signals"
                 msg %= self.command
             else: