rtmp and hls downloaders: Clarify error message when the external tools are not installed

Ask to install them, as we do in the postprocessor.
We get some reports with it, like #3061 or #3048.

youtube_dl/downloader/rtmp.py

@@ -106,7 +106,7 @@ class RtmpFD(FileDownloader):
         try:
             subprocess.call(['rtmpdump', '-h'], stdout=(open(os.path.devnull, 'w')), stderr=subprocess.STDOUT)
         except (OSError, IOError):
-            self.report_error('RTMP download detected but "rtmpdump" could not be run')
+            self.report_error('RTMP download detected but "rtmpdump" could not be run. Please install it.')
             return False
 
         # Download using rtmpdump. rtmpdump returns exit code 2 when