Hotfix/report registration failures

provisioner output was being redirected into a unnumbered provisioner log file.  This is the only place where errors during plugin loading/registration can be seen.
- [x] remove redirection so $LOOM_HOME/bin/loom-provisioner.sh register logs to STDOUT
- [x] some basic tracking of errors by the pluginmanager, so that proper exit code of --register can be determined.  

```
Waiting for server to start before loading default configuration...
Loading default configuration...
Registering provisioner plugins with server http://localhost:55054
2014-05-06 13:26:28 -0700 INFO: Successfully registered joyent
2014-05-06 13:26:28 -0700 INFO: Successfully registered openstack
2014-05-06 13:26:28 -0700 INFO: Successfully registered rackspace
2014-05-06 13:26:28 -0700 INFO: Successfully registered chef
2014-05-06 13:26:28 -0700 INFO: Successfully registered shell

Go to http://localhost:8100. Have fun creating clusters!
```