avconv: do not abort immediately if initializing hwaccel fails

exit_program() will try to free the decoders, which is not a good idea
from within get_format().
Return an error instead.

avconv.c

@@ -1507,7 +1507,7 @@ static enum AVPixelFormat get_format(AVCodecContext *s, const enum AVPixelFormat
                        "%s hwaccel requested for input stream #%d:%d, "
                        "but cannot be initialized.\n", hwaccel->name,
                        ist->file_index, ist->st->index);
-                exit_program(1);
+                return AV_PIX_FMT_NONE;
             }
             continue;
         }