Error handling improvements, comment cleanup

Adds a new error type (GdriveErrorPath) and the corresponding helper
functions to determine if an object was not found in g.Stat
(IsObjectNotFound). Changed all calls to reflect that and simplified
error returning a bit (in most cases, errors from g.Stat will be
surfaced directly, without adding a layer of fmt.Errorf inside
the high level functions.)

Also did more comment cleanup/standardization.
Fix bugs introduced by the new error handling.