fix failing hr_attendance_analysis tests

the tests would fail when run after the 11th of the month
because then the hr.contract created by the demo data would not be valid at the
time of the timesheet.

* change the start date of the contract in the tests
* fix `HrAttendance.get_default_calendar` to raise an exception when no contract
  is found instead of returning an empty list which caused an `AttributeError`
About date format in error message, I opened https://github.com/OCA/hr-timesheet/issues/11

For me, this PR can be merged, so that we get tests working