Fix exception on playing Flash S01E02 on The CW with subtitles

Exception when playing "The Flash" S01E02 on "The CW" with subtitles turned on. This is because for this episode there is a subtitle line that has no 'Text' metadata in the subtitle_line. Fixed by checking for 'Text' before processing the line.

Also added exception handling so that episodes will play in the event on an exception when loading the subtitles, and a dialog so that the user can see that something is happening in the second or so while the subtitles are being downloaded and processed.

Note that I have not seen this exeption on any other episode of any show on The CW.

The errant subtitle_line contains the following only:

{u'guid': u'89a5dde6-35e5-41af-a51a-e8924e4b13a4', u'subTrack': None, u'tags': [], u'track': u'Ad Cue Points', u'startTime': 1146.969696, u'hitType': u'tag', u'segmentTypeId': u'4cb53a3908b1cb506e00002a', u'endTime': 1146.969696, u'id': u'543826187d4e501cd7726c12', u'metadata': {u'duration': u'', u'adCount': u'', u'mobileDuration': u'', u'Target': u''}}