Fix exception string format
Fix exception string format

The string %(action) is not valid, it misses the conversion specified,
add s for string.

Note that this leads to an untranslatable string, since our translation
tools check for valid formats and fail. In this case the failure comes
from a source code fail.

Change-Id: I2e630928dc32542a8a7c02657a9f0ab1eaab62ff

stackforge/watcher,stackforge/watcher,openstack/watcher,openstack/watcher