Improve resilience of calendar service

[Pierre-Gilles]

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• If your changes affects code, did your write the tests?
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Description of change

Make the calendar service more strong by not crashing if the insert of one event fails

[codecov]

Codecov Report

Merging #975 (cbb0ced) into master (d14b141) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #975      +/-   ##
==========================================
+ Coverage   94.49%   94.51%   +0.01%     
==========================================
  Files         485      485              
  Lines        6435     6438       +3     
==========================================
+ Hits         6081     6085       +4     
+ Misses        354      353       -1     

Impacted Files	Coverage Δ
...services/caldav/lib/calendar/calendar.formaters.js	90.62% <ø> (-0.15%)	⬇️
.../caldav/lib/calendar/calendar.syncUserCalendars.js	100.00% <100.00%> (+1.85%)	⬆️

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