Improve the error message when returning a payload above the Lambda limits

[mnapoli]

Fix #1368, also reported in Slack.

Before, when returning a response above the Lambda limits (6MB), users would get the following logs:

• 413 error (payload too large)
• then 403 error when reporting the execution status (I am guessing this is because the execution is already failed by the previous failed call, so this is redundant and rejected by the Runtime API)

This PR improves this specific error so that it's much clearer:

On top of that, the PR also adds low-level details for other Runtime API errors. This lets us more easily debug/understand what went wrong. In our example, it helps us understand the second error (403):

Thanks to that, we now know that after a 413 we shouldn't call the "signal failure" API. That removes the second error:

Positive side-effect: the runtime is no longer in an unknown state (because of unknown errors) and it will not restart PHP-FPM + the whole runtime 💪

[mnapoli]

Still need to figure out why the execution is marked as successful in metrics 🤔

[mnapoli]

Still need to figure out why the execution is marked as successful in metrics 🤔

It turns out that's an intentional behavior, that's how the NodeJS runtime behaves.

It seems impossible (unless we catch size > 6MB but what if the limit changes?) to prevent, so we'll go with that.