Drive: possible infinite loop when a pair is blacklisted

If Synchronizer.error_skip_period if is too small, basically if it's lighter than the duration of an iteration of the while loop in Synchronizer.synchronize().

Visible in TestIntegrationLocalStorageSpaceIssue.test_synchronize_no_space_left_on_device by setting for example:

syn.error_skip_period = 0.001