Add ability to add error handling middleware

[jakeleventhal]

I want to be able to pass in a custom error handler that allows me to handle errors (create logs, send info to Sentry, etc.)

[Roman991]

I dont remember if I already send the PR to export the CustomError class, which catches errors in amazon-sp-api, btw u can do it by yourself.
export the CustomError and then check

if(YourError instanceof CustomError){
  ...
}