Improve error message due to node version mismatches

[Woodz]

Expected Behavior

A message that explains that Node (would be good to include version here as well) could not parse the handler code

Actual Behavior

  "errorType": "Runtime.UserCodeSyntaxError",
  "errorMessage": "SyntaxError: Unexpected token '.'",
  "trace": [
    "Runtime.UserCodeSyntaxError: SyntaxError: Unexpected token '.'",
    "    at UserCodeSyntaxError.ExtendedError [as constructor] (/opt/nodejs/node_modules/datadog-lambda-js/runtime/errors.js:113:28)",
    "    at new UserCodeSyntaxError (/opt/nodejs/node_modules/datadog-lambda-js/runtime/errors.js:147:42)",
    "    at _loadUserApp (/opt/nodejs/node_modules/datadog-lambda-js/runtime/user-function.js:108:19)",
    "    at load (/opt/nodejs/node_modules/datadog-lambda-js/runtime/user-function.js:148:19)",
    "    at Object.<anonymous> (/opt/nodejs/node_modules/datadog-lambda-js/handler.js:65:59)",
    "    at Module._compile (internal/modules/cjs/loader.js:999:30)",
    "    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)",
    "    at Module.load (internal/modules/cjs/loader.js:863:32)",
    "    at Function.Module._load (internal/modules/cjs/loader.js:708:14)",
    "    at Module.require (internal/modules/cjs/loader.js:887:19)"
  ]
}```

## Steps to Reproduce the Problem

  1. Create a Lambda handler in JS that uses node v14 and nullish-chaining (`?.`) but runtime is node v12
  1. Add DD layer for node v12 and set handler env var to our Lambda handler
  1. Test Lambda

## Specifications

  - Datadog Lambda Layer version: Datadog-Node12-x | 67
  - Node version: v12/14

## Stacktrace

Paste here

[astuyve]

Hi! Today with handler redirection, this is probably as good of an error message as we can get. Unfortunately we're require-ing the handler and this is the error generated when node can't parse the file.

I do think it's unfortunate that the stack trace flys through Datadog, but I think you can avoid that using NODE_OPTIONS to bootstrap this library following this method.

Thanks, hope I've helped!