drive: Don't return nil Object with nil error from newObject* functions.

Before this change the newObject* functions could return object=nil
with err=nil.  The result of these functions are passed outside of the
backend code (eg in Copy, Move) and returning a nil object with a nil
error leads to crashes elsewhere as it breaks expectations.

After this change we return (nil, fs.ErrorObjectNotFound) in these
cases. The one place this is actually needd internally (when turning
items into listings) we detect that error and use it to mean skip the
directory item.

This problem was noticed while testing the shortcuts code. It
shouldn't happen normally but it is conceivable it could.

backend/drive/drive.go

@@ -1281,16 +1281,16 @@ func (f *Fs) newObjectWithExportInfo(
 		return f.newRegularObject(remote, info), nil
 	case f.opt.SkipGdocs:
 		fs.Debugf(remote, "Skipping google document type %q", info.MimeType)
-		return nil, nil
+		return nil, fs.ErrorObjectNotFound
 	default:
 		// If item MimeType is in the ExportFormats then it is a google doc
 		if !isDocument {
 			fs.Debugf(remote, "Ignoring unknown document type %q", info.MimeType)
-			return nil, nil
+			return nil, fs.ErrorObjectNotFound
 		}
 		if extension == "" {
 			fs.Debugf(remote, "No export formats found for %q", info.MimeType)
-			return nil, nil
+			return nil, fs.ErrorObjectNotFound
 		}
 		if isLinkMimeType(exportMimeType) {
 			return f.newLinkObject(remote, info, extension, exportMimeType)
@@ -1801,7 +1801,11 @@ func (f *Fs) itemToDirEntry(remote string, item *drive.File) (fs.DirEntry, error
 	case f.opt.AuthOwnerOnly && !isAuthOwned(item):
 		// ignore object
 	default:
-		return f.newObjectWithInfo(remote, item)
+		entry, err = f.newObjectWithInfo(remote, item)
+		if err == fs.ErrorObjectNotFound {
+			return nil, nil
+		}
+		return entry, err
 	}
 	return nil, nil
 }