Allows to add non-existing media files

Submitted by Bastien Nocera @hadess

Link to original bug (#766231)

Description

From https://bugzilla.redhat.com/show_bug.cgi?id=1334770

Description of problem: Totem allows to add non-existing media files, which causes unnecessary error while trying to play such file

Version-Release number of selected component (if applicable): 3.14.3

How reproducible: Always

Steps to Reproduce:

1. open totem, click add local video
2. type path to non-existing location of the media file and press enter
3. click on the media file icon to play it

Actual results: An error occurred - The specified movie could not be found.

Expected results: Totem doesn't allow addition of non-existing media files

Additional info:

Version: 3.18.x