QPID-5721: Improve exception message used when String operand cannot be interpreted as JSON

* Also made AVC consistently return unmodifiable when converting from a JSON string

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1602078 13f79535-47bb-0310-9956-ffa450edef68
QPID-5812: python high level API client receiver.fetch(timeout=N) blocks forever in select() if broker is stopped (kill -STOP)

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1602221 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' into broker_options_decouple