Currently we log on every single one of 10 retries deep in the upnp
stack. However we also return the failure as an error, which is bubbled
up a while until it's logged at debug level. Switch that around, such
that the repeat logging happens at debug level but the top-level happens
at info. There's some chance that this will newly log errors from
nat-pmp that were previously hidden in debug level - I hope those are
useful and not too numerous.
Also potentially this can even close#9324, my (very limited)
understanding of the reports/discussion there is that there's likely no
problem with syncthing beyond the excessive logging, it's some weird
router behaviour.
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