Always raise on errors

[ksol]

This PR re-orients the gem towards raising exception when something goes wrong (while previously it was configurable).

This means raising exceptions on error responses, but also when the token is missing and when it is expired.

The code is smaller/simpler as a result, which is always nice.

Side tasks:

• token expiration is read directly from the token instead of assuming 1h (and leaving room to customize it). this adds a dependency on jwt
• addressable was missing in the dependencies of the gem (it was introduced a couple of PRs ago)
• some dead code/comments were still around
• changelog/readme was updated

This also marks the gem as 4.0 beta 1, as the largest internal work is now complete