ExternalFD:real_download(): Skip getsize/rename when writing to stdout

In the real_download() method of the ExternalFD class:

Special-case output to stdout, when filename=='-', because otherwise
the call to os.path.getsize() raises
  OSError: [Errno 2] No such file or directory: '-'
which in turn gets masked by a try block and re-raised as
UnavailableVideoError().

And even if getsize had succeeded, we would fail on renaming, and then
we would also fail on reporting progress as report_progress() expects
to have file size information.

So report download completion with self.to_screen() just as
report_progress() would in the noprogress==True case.

You might ask whether it's an abstraction violation for external.py to
have to know and think about '-' as a filename, but unfortunately
we're there already. _call_downloader() thinks about it, and also
real_download() calls common.py's temp_name() which special-cases the
'-' filename to preserve it instead of returnign a temp file. So we
already worry about this case at this abstraction level, as well as
both below and above. Perhaps that should change, but that's for
another day.

youtube_dl/downloader/external.py

@@ -27,15 +27,20 @@ class ExternalFD(FileDownloader):
 
         retval = self._call_downloader(tmpfilename, info_dict)
         if retval == 0:
-            fsize = os.path.getsize(encodeFilename(tmpfilename))
-            self.to_screen('\r[%s] Downloaded %s bytes' % (self.get_basename(), fsize))
-            self.try_rename(tmpfilename, filename)
-            self._hook_progress({
-                'downloaded_bytes': fsize,
-                'total_bytes': fsize,
-                'filename': filename,
-                'status': 'finished',
-            })
+            if filename == '-':
+                # xxx report_progress() expects total_bytes to be set or it throws a KeyError, so
+                # we can't just call: self._hook_progress({'status': 'finished'})
+                self.to_screen('[download] Download completed')
+            else:
+                fsize = os.path.getsize(encodeFilename(tmpfilename))
+                self.to_screen('\r[%s] Downloaded %s bytes' % (self.get_basename(), fsize))
+                self.try_rename(tmpfilename, filename)
+                self._hook_progress({
+                    'downloaded_bytes': fsize,
+                    'total_bytes': fsize,
+                    'filename': filename,
+                    'status': 'finished',
+                })
             return True
         else:
             self.to_stderr('\n')