More specific error classes

[mostlyobvious]

I'd be wonderful to be able to dispatch errors by class rather than by checking message they pass.

Currently:

when 401
  raise StreamApiResponseException, error_message(response, 'Bad feed')
when 403
  raise StreamApiResponseException, error_message(response, 'Bad auth/headers')
when 404
  raise StreamApiResponseException, error_message(response, 'url not found')

Having:

UrlNotFound = Class.new(StreamApiResponseException)

when 404
  raise UrlNotFound, error_message(response, 'url not found')

would be backwards compatible and enable responding differently when matching Stream::UrlNotFound === error in application codebase.

[Josiassejod1]

I appreciate this comment, had an issue trying to test for errors and having to guess the underlying implementation when it came to writing a spec @pawelpacana

[ferhatelmas]

fixed by #142, thanks @groyoh