Print error stacktrace

Hello, would you please make it possible to print stacktrace? I got many errors that are very hard to find

for example
```
[VWO] [activate] (vwo/vwo): activate API error: undefined method `[]' for nil:NilClass
```
when I call `vwo_client.activate(...)` it even return error object `#<NoMethodError: undefined method '[]' for nil:NilClass>`

please, this gem is hard to use