Recovery from errors on the new scheduler for Git

[sduenas]

The current scheduler - sirmordred - is not able to recover from errors. The only way to do it is manually (normally restarting the scheduler). There are some situations when this is even harder. For git, to be sure data wasn't lost, the user will have to remove the last git repository that was being analyzed from the perceval cache.

The goal is to add support on the new scheduler to recover from these type of errors. For this, the possible scenarios are:

• The code that fetches data fails
• The scheduler is shutdown manually while fetching data
• The scheduler shutdowns unexpectedly

This will be a common objective for the scheduler but, since we're starting to work only with Git data, this task is only related to that datasource.