Minor code cleanup

https://bugzilla.mozilla.org/show_bug.cgi?id=978670
Show correct error message if script file fails to open

https://bugzilla.mozilla.org/show_bug.cgi?id=978670
Fix drag events on Nexus 4

Press and release needs to use a new tracking id on this platform.

https://bugzilla.mozilla.org/show_bug.cgi?id=995895