ipa-epn: don't use too general exception

When modifying ipa-epn code, a warning was issued:

--------------
Python 3.11.4 (main, Jun  7 2023, 00:00:00) [GCC 13.1.1 20230511 (Red Hat 13.1.1-2)]
************* Module ipaclient.install.ipa_epn
ipaclient/install/ipa_epn.py:89: [W0719(broad-exception-raised), drop_privileges] Raising too general exception: Exception)
--------------

Use 'RequiresRoot' exception class and clarify the message:
    ipalib.errors.RequiresRoot: Cannot drop privileges!

Related: https://pagure.io/freeipa/issue/9425

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>

ipaclient/install/ipa_epn.py

@@ -86,7 +86,7 @@ def drop_privileges(new_username="daemon", new_groupname="daemon"):
         os.setuid(grp.getgrnam(new_groupname).gr_gid)
 
         if os.getuid() == 0:
-            raise Exception()
+            raise errors.RequiresRoot("Cannot drop privileges!")
 
         logger.debug(
             "Dropped privileges to user=%s, group=%s",