erc-sh-archive: add diag message if backend is missed

bin/erc-sh-archive

@@ -33,6 +33,7 @@ set_backend()
 		# TODO: try install patool and p7zip
 		is_command 7za && HAVE_7Z='7za'
 		#is_command 7z && HAVE_7Z='7z'
+		is_command $HAVE_7Z || fatal "Could not find any patool or 7-zip backend. Please install p7zip package and retry."
 	fi
 	[ -z "$force" ] || HAVE_7Z="$HAVE_7Z -y"
 }