Compute default error duration if omitted

[pederhan]

This PR adds a computed default for a source collector's error_duration based on its update_interval and error_tolerance.

Given a the following config, we can expect ZAC to compute a default value for error_duration that is between 300 and 359 seconds:

[source_collectors.faultysource]
module_name = "faultysource"
update_interval = 60
error_tolerance = 5
exit_on_err = false

This PR uses the following formula to compute the default value: round(error_tolerance * update_interval + (update_interval*0.9)). So for the config above, we will get a default error_duration of 354.

The extra duration (update_interval*0.9) allows for some accumulated latency during and between updates, while still ensuring 5 successive errors are required to trigger deactivation.