Fix vm start UI error return message

This patch fixed the UI error return message and also fix the button
name used in the JS function.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Fix VM stop error messages handling (backend/UI)

The UI was not able to catch the error messages sent from backend when a
user tried to stop a VM. This patch fixes the processes and also change
the 'stop' backend function, then exceptions from "get_vm" are send to
frontend.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Fix VM reset (UI) error messages hnadling

This patch fixes the UI in order to show properly the messages sent by
backend when usert tries to reset a running VM. Error messages are
basically sent from vm 'stop' and 'start' functions.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Fix VM delete error message handling (UI/Backend)

This patch fixes delete confirmation window, changes backend in order to
raise proper exception and fixes the ui to catch the right message from
backend.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Delete unsed _vm_exists funtion

Due to the new error messaging handling the function _vm_exists is not
necessary anymore and moved OperationFailed exception to get_vm function,
where makes more sence.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>
Fix Kimchi vm tests according to new messages sent from backend

With new backend messaging system, when a VM is not found the backend
raises an exception which was breaking Rollback functions. This patch
fixes this problem.

Signed-off-by: Rodrigo Trujillo <rodrigo.trujillo@linux.vnet.ibm.com>