Replace failure handling with up-front check.

The only time that write_string should fail is if the Python is completely braindead.
Check for that condition and output a more accurate warning.

See #3326 for details.
[shared] Add extractor (Closes #3312)
[youtube] Fix player ID display
[jsinterp] Allow uppercase object names
[jsinterp] Add new testcase
[jsinterp] Fix slice
release 2014.07.25
[extractor/common] PEP8
[gdcvault] Add support for direct URL video type

Fixes #3356
release 2014.07.25.1
[livestream] Parse SMIL (#2713)
[justin.tv] Fix page reporting (#3352)

youtube-dl -j http://www.twitch.tv/fang_i3anger still fails though.
Merge branch 'master' of https://github.com/rg3/youtube-dl