Description of problem:
Creating route with ack value higher than UINT16 throws invalid conversion instead of range error describing that user exceeds the max value allowed in the context.

Version-Release number of selected component (if applicable):
qpid-cpp-0.18-14
qpid-cpp-0.18-16
qpid-cpp-0.22-4

How reproducible:
100%

Steps to Reproduce:
1. create allowing acl
  acl allow all all
2. start broker with this acl loaded
3. prepare exchange and queue for route
  qpid-config add queue q
  qpid-config add exchange topic e
4. create a queue route with --ack 100000
  qpid-route --ack 100000 queue add e q '#'

Actual results:
Failed: Exception - invalid conversion: Cannot convert from int64 to uint16 (/builddir/build/BUILD/qpid-0.22/cpp/src/qpid/types/Variant.cpp:300)

Expected results:
Exception that explain the cause of error like 'the max allowed value for ACK is 65535' is thrown.

Additional info:
this problem will probably be in others numeric options.