Fix a recent regression in e17392a where JsonSetValue() stopped working (probably other functions as well).

Fix is to call through to the no-arg constructor from the string constructor in JsonStore, which I suspect was just forgotten.
This was actually picked up by the TestJsonSetValue() regression test failing
But this isn't being run on jenkins due to the .net version issue.
This commit also puts the full stack trace in logged messages and makes these error level messages instead of info
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim