eleven: fix invalid srt formats

Check for invalid time formats in srt files too. If there is an exception loading the file
just throw out the entire file.

Change-Id: Ic6011f843d1c433e9d8d84281c1c98dc5deaf547
JIRA: NIGHTLIES-1552
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>