Add error status to detect when an EVC had problems

[Alopalao]

The error Service Unavailable cannot be avoided. With enough number of requests, it will eventually happen.
This area is to deal with this and any other error while installing/deleting flows.

Sending more requests when a path fails is not ideal. Instead, we could implement a new field for the errored EVC.
error_status field will be set as:

{"error_status": {"current_path": "install", "failover_path": "delete"}}

New basic mechanic for path handling

In the case of path installation:

1. Delete the existing path only if it exists. Avoid to send requests when there is no path.
   1.1. Path deletion errored, log error, set error in status_error, leave previous path unchanged, sync
   1.2. Return
2. Clean up error_status
3. Create flows and send requests to install the path
   3.1. Path deletion errored, log error, set error in status_error, set new path, sync
   3.2. Return
4. Set new path
5. Return

In the case of path deletion:

1. Get the previous path to be deleted if current_path or failover_path is being deleted
   1.1.1. Path does not exist
   1.1.2. Return
2. Create flows and send requests
   2.1. Requests errored, log error
   2.2. Raise exception EVCPathNotDeleted (New exception)
3. Return

Additional changes

• Add exception EVCPathNotDeleted to be raised when a deletion error happens.
• Handle exception EVCPathNotDeleted when deleting EVC. If an error happens, the EVC will not be deleted. It will just be disabled and deactivated.
• Add UI element to identify EVC with status_error, issue UI: Show that an EVC has an error #510
• Add request to deal with EVCs with status_error, issue Error handling when installing and deleting flows #495. More information in the next label