LanguageWorkerChannel Error handling

We recently had an issue in the worker channel where malformed JSON bodies would cause hangs. This was due to poor error handling in the channel code, addressed by PR https://github.com/Azure/azure-functions-host/pull/2918.

That PR fixed a specific issue, but from this issue we've learned that any unhandled exceptions in this code can cause hangs. Therefore, we need to put general error handling in place in this code, test all the various error paths and make sure we have proper backstops to ensure no hangs are possible.