Error in startNew should print a useful message

Currently it's printing `Error` without any details. Examples:

* https://github.com/Azure/azure-functions-durable-js/issues/90#issue-454307747
* https://github.com/Azure/azure-functions-durable-js/issues/90#issuecomment-545149464

If possible, inspect other locations where errors are thrown and ensure they print the error message to logs as well.

Locate the startNew entry point and trace where its errors are caught or logged. Then inspect the other error-throwing locations mentioned by the issue and run the relevant tests or examples; done means logs include useful error details instead of only `Error`.

Written by the indexing model from the issue text.