started extracting UI logic from coub_dl class

coub-dl.py

@@ -31,6 +31,10 @@ class File(Enum):
     OUTPUT = 4
 
 
+class DownloadFailure(RuntimeError):
+    pass
+
+
 class coub_dl:
     default_files = {Stream.AUDIO: 'audio', Stream.VIDEO: 'video',
                      File.LIST: 'list.txt', File.LOOP: 'loop', File.FRACTION: 'fraction',
@@ -49,8 +53,7 @@ class coub_dl:
         self.download_audio_stream()
         self.download_video_stream()
         self.read_extensions()
-        try: self.check_downloads()
-        except RuntimeError: exit('Failed to download streams! This usually happens when Coub changes something.')
+        self.check_downloads()
         self.fix_video_stream()
 
         # get stream lengths via ffprobe
@@ -108,7 +111,7 @@ class coub_dl:
     def check_downloads(self):
         check = {Stream.VIDEO, Stream.AUDIO}
         if not all({exists(self._files_dict[item]) for item in check}):
-            raise RuntimeError('Download failed!')
+            raise DownloadFailure()
 
 
     def fix_video_stream(self):
@@ -222,4 +225,5 @@ if __name__ == '__main__':
 
     for url in set(args.URLs):
         print_opt('\nCreating video from {}'.format(url))
-        run(url, args.output, args.extension)
+        try: run(url, args.output, args.extension)
+        except DownloadFailure: exit('Failed to download streams! This usually happens when Coub changes something.')