Making it possible to access the details of error responses from the API (see #4)

[11BelowStudio]

Here's an implementation for #4.

If a NeoCitiesApiError was built from a BaseResponse instance, that response will now be stored in the NeoCitiesApiError instance, and can be accessed later on. If it was built from a Throwable, that field will just be null.

The existing error message building logic, and the now-redundant public NeoCitiesApiError(final String message) constructor have been retained for sake of consistency.

[nelson-mig-l]

Hi Rachel.
Thanks for pointing this out.
I will probably make the distinction between an:

• Error - reported by the api, always created from a server response
• Exception - some unexpected error on client side

Just to avoid having to deal will null checks. I will spend some time on this soon.

[11BelowStudio]

I've had a cunning plan. The getResponse() method now wraps the BaseResponse in an Optional<BaseResponse>, which should make things a little bit safer.

(The alternatives would probably be to change the ask method to either not wrap any IOException in a NeoCitiesApiError (meaning that all the methods of this library would need to have throws IOException on them), or to wrap the IOException inside a different exception, such as an UncheckedIOException (which you might want to document)).