MatroskaExtractor: Letting the error be correctly handled

Do not blow up when parsing the header fails (like, say, a
corrupt or zero-byte file) and let the error handling that's
already in place exit the extractor cleanly

Change-Id: I457574d4279efdd3a08e159e755b563d6d98fbc6
Merge pull request #3 from CyanogenMod/cm-11.0

MatroskaExtractor: Letting the error be correctly handled