Include stacktrace in error bodies

[mdneuzerling]

AWS Lambda error endpoints (both the initialisation error endpoint and the invocation error endpoint) expect a content type "application/vnd.aws.lambda.error+json" with the following body:

{
    "errorMessage": "...",
    "errorType": "...",
    "stackTrace": [],
}

The post_lambda_error function is responsible for formatting errors and POSTing them to the endpoint. However, it currently passes an empty stacktrace. The stackTrace must be a list of strings (character vector).

Simply including the call stack in a tryCatch block is insufficient here, as it won't capture the detail of the call stack in the "try block". The try_capture_stack from the evaluate package might be useful here (if I can work out how to use it).

Including these stacktraces will complicate the unit testing, as the stack will include a substantial depth of function calls made by testthat. Perhaps we can trim those somehow?