Error notifications, checksum & initial scan fixes

Improvements for all users:

• Starting from now, we will be able to automatically receive notifications of
  errors encountered by our users. Error reports won't include user logs in
  order to respect your privacy. We'll still have to ask you individually to
  send us our logs in order to investigate complex issues. But this should help
  us figuring out which errors occur the most, since not so many people can
  find the time to report them. For now, the notifications are only sent for a
  few hard-to-investigate issues, but we'll progressively send more of them.
• Checksums for new local files were computed but not sent during the initial
  upload, possibly delaying detection of content issues. Still the checksum was
  computed by the Cozy and any issue would've be detected during next polling.
  They will now be effectively detected as soon as they should be.

Improvements for macOS users:

• On devices automatically migrated from HFS+ to APFS while upgrading to macOS
  High Sierra, the initial scan was mistakenly detecting some files as conflict
  due to the new way we're detecting case/encoding conflicting files. This
  should now work as expected. This issue could theoretically have happened on
  both macOS & Windows in rare cases of weird local filename changes.

See also known issues.

Happy syncing!