g errors

When a Twilio Function deemed an async function throws an error, the
handler doesn't catch it and instead throws an unhandled promise
rejection error.

Also, when constructing the context and the checks for valid account
sids and auth tokens in the getTwilioClient function, we say that it
should print the error. However, passing the logger to the context
serializes and deserialises it, since it is being passed to another
process, and and it is no longer an instance of Logger. This causes
another error, trying to call  on the logger's error function that no
longer exists.

So, this PR does 2 things, it checks the handler function to see if it
is an async function and calls it with await so that it can catch the
error. And it sets shouldPrintMessage to true only if there is a logger
object that has an error function.

…unction-runner