app: reset source pending status upon unexpected source deletion at server

[redshiftzero]

Description

Fixes #966.

Test Plan

0. First add to the source deletion server API endpoint (above this line):

+            early_fail = random.choice([True, False])
+            if early_fail:
+                abort(404)

such that you get failures half the time.

1. Delete a source locally.
2. Upon deletion success, the pending status works as expected: the pending status is shown and the source disappears from the UI.
3. Upon deletion failure (not due to timeouts), an error message is shown, the pending status is removed from both the SourceWidget in the source list and the conversation wrapper.

Checklist

If these changes modify code paths involving cryptography, the opening of files in VMs or network (via the RPC service) traffic, Qubes testing in the staging environment is required. For fine tuning of the graphical user interface, testing in any environment in Qubes is required. Please check as applicable:

• I have tested these changes in the appropriate Qubes environment
• I do not have an appropriate Qubes OS workstation set up (the reviewer will need to test these changes)
• These changes should not need testing in Qubes

If these changes add or remove files other than client code, packaging logic (e.g., the AppArmor profile) may need to be updated. Please check as applicable:

• I have submitted a separate PR to the packaging repo
• No update to the packaging logic (e.g., AppArmor profile) is required for these changes
• I don't know and would appreciate guidance

[rmol]

👍