Adding functionality to catch panics that occur both inside the runtime handler and in the Future it returns

[fermanjj]

Description of changes:
Previously, if the lambda runtime handler Future panicked, then this would not be caught. These change add the functionality to catch those panics.

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[calavera]

Sorry it took me a while to review this PR. Thanks for adding a test, it makes things easier to follow. Have you bumped into any situation where this is helpful? I'm wondering if it'd be a good idea to document the panic behavior in general.

[fermanjj]

Thanks for taking a look! I've added in your suggestion and updated the commit. As far as running into this before, we've only noticed by looking at the implementation that the way panics would be caught wasn't handling panics inside an async block. Having this here is helpful when running concurrent handlers within a single runtime and not having a single thread kill the rest.