Show relation name in status when a relation hook fails

This change takes advantage of new information that's been recently
added to the FullStatus API.

Fixes LP #1194481
cmd/juju: refactor status formatting

Status formatting functions are now part of a struct. This avoids the
need to pass the relations map through many functions when it is only
used in one (deeply nested) place.
Merge pull request #109 from mjs/1194481-relation_name_in_status_when_relation_hook_fails

Better status information when a relation hook fails

The first commit makes a  bunch of changes which take advantage of new data now available in the FullStatus API response to show the remote endpoint details when a relation hook fails. This addresses LP #1194481.

The second commit refactors the status formatting code to make some of the changes made in the previous revision somewhat cleaner.