fix(runtime-handler): add warning for optional context vars

[dkundel]

I'm going with Option 2 from #304 here as it got the most upvotes. We'll release it as @twilio/runtime-handler/1.1.2 but in theory Option 2 is compatible with any version of the runtime-handler.

The warning looks the following:

re #304

Contributing to Twilio

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

[philnash]

Who doesn't love a regex over a a stack trace from an error that doesn't actually get thrown!