Proper error messages

[cblp]

Distinguish these cases in error messages:

• a document doesn't exist
• a document is invalid
• a document is tracked
• a document is archived

[willbasky]

Is it about own error type or more?

[cblp]

This is about UI.

What we have now:

$ ff show foo
ff: Error "Document with id note/foo has not found." []

What a user expects:

$ ff show foo
ff: Document with id note/foo has not found.