Weird error messages when using ff* wrappers

[rbouqueau]

For example I can start and playback an srt:// stream, but these errors are confusion:

$ gpac avgen c=avc c=aac -o srt://127.0.0.1:9000::ffmt=mpegts:mode=listener -graph
[FFMux] Fail to write packet to srt://127.0.0.1:9000: error Input/output error - aborting <= HERE
session last process error I/O Error                                                      <= HERE
Unknown argument ":9000" set but not used by any filter - no matching argument found      <= HERE

Filters connected:
 "avgen" (js=/home/rbouqueau/works/gpac/gpac/share/scripts/jsf/avgen/init.js) (idx=1)
-(PID audio) resample (dyn_idx=6)
--(PID audio) ffenc "ffenc:aac" (c=aac) (idx=3)
---(PID audio) ffmx (dst=srt://127.0.0.1:9000::ffmt=mpegts:mode=listener) (idx=4)
-(PID video) ffenc "ffenc:libx264" (c=avc) (idx=2)
--(PID video) rfnalu (dyn_idx=5)
---(PID video) ffmx (dst=srt://127.0.0.1:9000::ffmt=mpegts:mode=listener) (idx=4)
[aac] 2 frames left in the queue on closin

and (playback):

$ gpac -play srt://127.0.0.1:9000
Type 'h' in window for command list
Filter ffdec:9000 option set but not use <= HERE