decoder: make vout creation failure recoverable

The decoder can request another format, or even retry the same format
with better luck next time. So do not set b_error. b_error is for
unrecoverable failures, such as if the decoder module is upset.

src/input/decoder.c

@@ -415,7 +415,6 @@ static int vout_update_format( decoder_t *p_dec )
         if( p_vout == NULL )
         {
             msg_Err( p_dec, "failed to create video output" );
-            p_dec->b_error = true;
             return -1;
         }
     }