FIX: BitstreamConverter: don't silently close the converter in case of conversion error

@huceke Not sure who to assign this as we lost history;)

This patch just silently ignore conversion errors (beside logging an error).
Before, the error was silently ignored AND the converter was silently closed.

There might very well a better solution to handle this...
The context is that, apparently, the first frame after a h264 rtmp seek is not correct from a BitStream perspective. 
Could be a librtmp bug or something I miss...