Define typical document errors

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Document Error Responses are only given as plain strings. Clients may want more specific information about the type of document error, for instance to provide their own translation or to react based by providing the user different kinds of actions (e.g. search for another document). To do so, a list of typical document errors is required. These document error types could be provided for instance by an error code. Document error types, however, would be optional, so clients must also be able to handle general document errors as it is specified now.

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Possible general error cases for document errors (request/renew/cancel/items):

• inactive acount state: expired, outstanding fees... (see method patron for details)
• method not possible with this document: e.g. renew a document that is not held by the user
• patron quota exceeded: because maximum number of requests/renewals per patron is limited
• document quota exceeded: because maximum number of requests/renewals of this document limited
• method not allowed: e.g. renew a document that cannot be renewed for some reason
• system error: backend system did fully act as expected
• unknown URI: e.g. requested document or storage not found

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May be part of #76.