Mainly for debugging. Usually, we just set options for all possible
protocols, and we can't really know whether a certain protocol is used
beforehand. That's also the reason why avio_open2() takes a dictionary,
instead of letting the user set options directly with av_opt_set(). Or
in other words, we don't know whether an option that could be set is an
error or not, thus we print the messages only at verbose level.
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