Handle non-existent files/directories gracefully

The stacktrace from calling Find on an invalid directory is misleading,
as seen in #23. This catches the error and displays a warning message,
since we shouldn't be barfing on invalid user input.

Change-Id: Ib593011bb11e6a94e4ea3d32bcdfa4012f3e70d6
Reviewed-on: https://gerrit.causes.com/23314
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>