try prepending "string:" to tales expression if talesCompile fails

ISSUE - traceback seen:
```
2014-10-08 05:18:34,404 ERROR zen.actions: Path element may not be empty in '/opt/zenoss/bin/zensendevent -d test-rhel6.zenoss.loc -s Critical -c /Status "Test Automated Trigger"': /opt/zenoss/bin/zensendevent -d test-rhel6.zenoss.loc -s Critical -c /Status "Test Automated Trigger" context = None ...
```

regression from https://github.com/zenoss/zenoss-prodbin/commit/04dbe76961f7ae972e4ee05bec601e4208f710f7:
```
- return talesEvalStr(sourceStr, context, kwargs)
+ return talesEval(sourceStr, context, kwargs)
```
https://github.com/zenoss/zenoss-prodbin/blob/develop/Products/ZenUtils/ZenTales.py#L28-L32

DEMO:
```
# plu@plu-9: serviced service attach zenactiond su - zenoss
Last login: Wed Oct  8 20:35:39 BST 2014
(zenoss)[zenoss@29403a2a01d2 ~]$ /opt/zenoss/bin/zensendevent -d test-rhel6.zenoss.loc -s Critical -c /Status "TestAutomationEvent"

# plu@plu-9: serviced service attach zenactiond tail -F /opt/zenoss/log/zenactiond.log
2014-10-08 19:35:24,849 INFO zen.queueconsumer: Queue consumer ready.
2014-10-08 19:35:47,224 INFO zen.zenactiond: Event:'test-rhel6.zenoss.loc||/Status|5|TestAutomationEvent' Trigger:TestSimpleTrigger Action:command Status:SUCCESS Target: Info:<NotificationSubscription Info "TestSimpleNotification">
2014-10-08 19:35:47,353 INFO zen.actions: Command TestSimpleNotification finished successfully (exit code 0) with the following output:  
```