all: clean-up usage of GError

Functions that take a GError** MUST fill it in on error. There is no
need to check whether error is NULL if the function it was passed to
had a failing return value.

Likewise, a proper GError must have a non-NULL message, so there's no
need to double-check that either.

Based-on-patch-by: Dan Winship <danw@gnome.org>

src/dhcp-manager/nm-dhcp-dhclient.c

@@ -380,8 +380,8 @@ dhclient_start (NMDhcpClient *client,
 		if (!success) {
 			nm_log_warn (log_domain, "(%s): failed to save DUID to %s: (%d) %s.",
 			             iface, priv->lease_file,
-			             error ? error->code : -1,
-			             error && error->message ? error->message : "(unknown)");
+			             error->code,
+			             error->message);
 			g_free (pid_file);
 			return FALSE;
 		}
@@ -591,8 +591,8 @@ get_duid (NMDhcpClient *client)
 		if (error) {
 			nm_log_warn (LOGD_DHCP, "Failed to read leasefile '%s': (%d) %s",
 			             priv->def_leasefile,
-			             error ? error->code : -1,
-			             error ? error->message : "(unknown)");
+			             error->code,
+			             error->message);
 			g_clear_error (&error);
 		}
 	}