Show proper dialog for unavailble streams

Without this the following exception occurs.
 Error Type: <type 'exceptions.NameError'>
 Error Contents: global name 'errormsg' is not defined
 Traceback (most recent call last):
   File "/storage/.xbmc/addons/plugin.video.svtplay/default.py", line 369, in <module>
     startVideo(ARG_URL)
   File "/storage/.xbmc/addons/plugin.video.svtplay/default.py", line 297, in startVideo
     if not errormsg:
 NameError: global name 'errormsg' is not defined