Currently all errors during pulling and the first of these errors again on
finishing are logged to info. Besides that the errors logged when finishing
are stored in f.errors. This PR moves all logging during pulling to the debug
channel (they might still be relevant in some obscure debugging case) and
uses the stored errors to log the main error per fail when all pulling
iterations are done and failed.
Additional instead of trying 11 times it now only tries 3 times.
This is the first part of what is discussed here:
https://forum.syncthing.net/t/reduce-verboseness-of-puller/10261
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4338