charmstore/router: generate http.StatusNotFound appropriately

This generates messages (and error codes) as expected by the API specification.
This tests are getting harder to read (might be just the hour of day), We have two same urlStrings, but different codes. The test with the constMetaHandler(nil) returns ErrMetadataNotFound is clear, however, it is "obvious on first sight" why previous test returns ErrNotFound instead (as no handler is provided).