[RFE] Improve Error Handling For Subscribe

[mitchwadair]

Is your feature request related to a problem? Please describe.
As described in #57, it is pretty unclear why a subscription would fail to be made.

When making a request to subscribe without proper authorization, the message looks like so:

{
  error: 'Forbidden',
  status: 403,
  message: 'subscription missing proper authorization'
}

When making a request to subscribe and there is already and existing subscription that matches the type and condition, the message looks like so:

{
  error: 'Conflict',
  status: 409,
  message: 'subscription already exists'
}

TESjs is too optimistic in assuming developers will know to give correct auth scopes to their client, and that subscriptions are cleaned up and so passes the data naively to the subscription queue assuming it was OK.

Describe the solution you'd like
Provide a more clear error on a non-OK response.

Additional context
#57