Fix API Gateway error responses

[adambrgmn]

As earlier discussed in issue #186, I took the liberty of (hopefully) updating all aws-node-... examples with lamdas that responds to API Gateway-events and changed their error responses so that they respond correctly.

I also want to point out that I haven't changed the structure of the code or anything. I have only changed the original callbacks, e.g.:

if (error) {
  callback(new Error('Something went wrong'));
}

I have transformed them so that the lambdas ”don't think” that an error occured, e.g.:

if (error) {
  callback(null, {
    statusCode: error.statusCode || 501,
    headers: { 'Content-Type': 'plain/text' },
    body: error.message || 'Internal server error',
  });
}

In the first case an http request would always return a 502 Bad Gateway response. But in the latter case the response would be more correct and give useful feedback to users.

I have tried to keep the errors as close to the original as possible, and sometimes I have guessed on a suitable statusCode and message.

I hope that it is okey that I made this into a larger PR, instead of splitting it up into one PR per example.

This PR resolves #186.

[christophgysin]

Thanks!