Fix b/13673493: Better warning message if start path is not shared.

Currently, if a folder on a server is not shared, the adaptor generates
the warning:
"The path ... is not a valid path. The path does not exist or it is not
 a file or directory."

If a folder on a server is not shared, the adaptor has no idea if it
exists or not.  This change simply changes the error message slightly:
"The path ... is not a valid path. The path does not exist, or it is
 not shared, or it is not a file or directory."

This phrasing leaves open the possibility of supporting local file
systems.

Code Review: http://codereview.appspot.com/86630044
1 file changed