Improved the error handling.

[Jack12816]

When an error occurred deep in the called factory the API endpoint responded something like this:

ActionView::MissingTemplate:
  Missing template factory_bot/instrumentation/root/create,
  factory_bot/instrumentation/application/create with {:locale=>[:en],
  :formats=>[:html], :variants=>[], :handlers=>[:raw, :erb, :html, :builder,
  :ruby]}. Searched in:
    * "/app/spec/dummy/app/views"
    * "/app/app/views"      

Now the error handling can be customized and the default error response looks like this:
(HTTP status code: 500)

{
  "application": "Name of your Application",
  "error": "Factory not registered: \"admin\"", 
  "backtrace": "[..]"
}