Publish JOhm error-dictionary

[gsharma]

We should publish an error dictionary of all the errors that can be thrown as JOhmException's so that users can setup mechanisms to be able to explicitly respond to a particular error scenario.

I think something like a JOhmExceptionMetadata enum can do the job easily. It can encapsulate a running error-number and a message. JOhmException further wraps this object and we ditch the single string constructor in favor of this new one. getMessage() can just delegate to getMetadata().getMessage() in order to not break existing client integrations.

We should also publish this error code:message list on the wiki for our users' easy reference.