Handle errors

[tonisuter]

From https://docs.aws.amazon.com/lambda/latest/dg/runtimes-custom.html:

Handle errors – If an error occurs, call the invocation error API.

[NSCoder]

Hey @tonisuter!

Thank you for building this, is amazing! I wanted to help with this issue but it seems that you already got it done:

aws-lambda-swift/Sources/AWSLambdaSwift/Runtime.swift

Line 65 in 2114940

func postInvocationError(for requestId: String, error: Error) {

Is that correct? Otherwise please point me in the right direction and I will submit a PR