Improve error messages responded by APIs

[adrianliaw]

Currently, the error message returned by the APIs are mostly "Target document not found", but it should be more specific. For example, when we try to look up incidents of an app but there's nothing, it always responds "Target document not found", but the fact that there's no incident found might be because the app is healthy, or, there's no app metric data found in the database. We should have analyzer telling these messages somewhere.