Improve usability of response specifications

[adamw]

Closes #1771

Changes:

• introduce asXyzOrFail methods which throw exceptions for non-2xx responses
• add asJsonOrFail, asJsonOrFailEither variants to all integrations
• quickRequest now uses asStringOrFail as the default response type
• scaladocs & docs updates
• move .mapRight, .orFail to ResponseAs for better discoverability (instead of extension methods), however this causes that type parameters have to be provided explicitly sometimes

cc @bishabosha @lbialy