Catch InstanceIsLocked exception on server actions

In v3, if the server is locked, a 409 error is raised, and the error msg
is like "the server is locked". In v2 now, the 409 error is raised too. But the
error msg is like "Instance is in an invalid state for \'pause(action name)\'".
The error msg is inappropriate, two points:
1. the state is valid for pause
2. the message doesn't tell that the instance is locked

The patch fixes the ploblem, and will do as v3. See the v3 commit:
2c9fca6a3c916f3f594ddbb1a8ee8946b8ea3ac7

Change-Id: I76572e37af686a4af35c31a3ea27f7070894b88a
Closes-Bug: #1255386
Merge "Catch InstanceIsLocked exception on server actions"