Always catch exceptions as const

All exceptions should be caught as const. With inconsistent use of const/non-const, rethrown exception is not caught in non-const handler. I am not so sure about the above, the problem (actually HRCPP-119) was in fact caused by HotRodClientException not inheriting from Exception.

Also, for rethrowing exceptions use throw; (not throw e; - this causes the exception to be sliced to the caught type, blocking polymorphism)