Feature: Add AssociationError

This error will be used when we have an issue associating one resource with
another.  For example, if we have a `Plan` and a `Subscription`, and there is no
plan with an ID of 123, if we attempt to associate a subscription with plan 123,
then it could potentially raise this type of error.

The first person to complain that the developer details keys isn't underscored
will get 30 lashes with a Singaporian cane (PS: Make sure Blake brings back
a Singaporian cane.) (PSS: Make sure Blake comes back period.)

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Change-Id:     I0de18b1569ded3debd700a8ae825e4695bf82da6
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Bugfix: Remove a manual reference to `knowledgebase_article_id`

We changed this a while back to be performed in the base class, we just missed
removing this one.

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Change-Id:     I98ec5a71a704fbc7e98ac0bc97816fd2368bad02
Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>
Version Bump to 1.6.0