String change in gnome-desktop

[Vincent Untz <vuntz gnome org>]

Hi,

I changed "Error cannot find file id '%s'" to "Cannot find file '%s'" to
fix https://bugzilla.gnome.org/show_bug.cgi?id=610568

This is only an error string, so no impact on documentation (hence no
gnome-doc-list in cc)

Thanks,

Vincent

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