Print traceback when raising UnavailableVideoError

An OSError or IOError generally indicates something a little more
wrong than a "simple" UnavailableVideoError, so print the actual
traceback that leads to the exception. Otherwise meaningful postmortem
debugging a bug report is essentially infeasible.

youtube_dl/YoutubeDL.py

@@ -1701,6 +1701,7 @@ class YoutubeDL(object):
                 self.report_error('unable to download video data: %s' % error_to_compat_str(err))
                 return
             except (OSError, IOError) as err:
+                traceback.print_exc()
                 raise UnavailableVideoError(err)
             except (ContentTooShortError, ) as err:
                 self.report_error('content too short (expected %s bytes and served %s)' % (err.expected, err.downloaded))