EC2 error handling updated so correct codes / messages can be used

ComputeException is added for server errors and ClientComputeException
for client errors. ReplyQueue and RestfulMarshallingHandler are updated
to marshall these errors to the correct format.
EC2 compatible error for group deletion failures - EUCA-1247

Deletion that fails due to group in use now results in an error response
with status code 400 and the following code / message:

  InvalidGroup.InUse
  Specified group cannot be deleted because it is in use.

Lookup for groups using invalid name or group identifier now result in
error responses using the code / message:

  InvalidGroup.NotFound
  The security group 'XXX' does not exist

where XXX is the group name or identifier.

EUCA-1247
EUCA-2655
EUCA-5840