Show user-friendly error for client/server incompatibility

Currently, razor-client version 0.15.0 cannot run commands with
versions of razor-server prior to 0.15.0, yielding an unexpected
404 from all commands.

So this change is twofold:
1) Supply a nice message when versions are incompatible, and
2) Allow `--json` as a way to bypass this incompatibility.

Fixes https://tickets.puppetlabs.com/browse/RAZOR-278