Make VPNaaS 'InUse' exception more clear

In case IpSecPolicy or IKEPolicy is updated while VPN connection that uses it
is already established, IPsecPolicyInUse or IKEPolicuInUse is raised.
Need to clarify their messages to emphasize that policies can't be updated
because they are used by established connection.

Change-Id: I259f9b8bcff7f8ec13ac630285f6e881c6653309
Closes-Bug: #1315809
Merge "Make VPNaaS 'InUse' exception more clear"