DOC: Added error code details for package validation

[carlwilson]

For document and package errors there are now:

• ID codes that correspond to the messages given by the application;
• the text of the error from the resource file;
• a summary of the specification text; and
• a reference to the appropriate ODF Specification and section.

TODO: XML validation errors need work to allow them to apply to multiple file types.

Addresses some of #41