Fix potential crash situation WRT queue.

[ntoll]

Description

Found while fixing #716. Perhaps addresses #662?

If the queue has an upper limit (like the metadata queue does), then the re_add_job method may try to do something that results in a queue.Full exception being thrown. This can be safely ignored. Because this case wasn't handled, I encountered an application crash while investigating #716 which led to this fix.

In addition I noticed there being a possibility of jobs being put onto the metadata queue if the user wasn't logged in (something which shouldn't happen). This has also been fixed.

Test Plan

Unit tests updated to reflect both changes.

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

[sssoleileraaa]

This doesn't fix #662 (https://user-images.githubusercontent.com/213636/70762572-d5f70880-1d05-11ea-97c0-f7e2c9b086a5.png) because we will still update the error bar in the client to say we can't connect to the server when the api token is None. The fix for #662 is in this branch: when-token-invalid-user-must-log-back-in which will be a PR once adequate tests have been added and the bug is confirmed to be fixed. If you're not convinced, you can easily test whether or not your changes fix this issue by logging out when there are items left in the queue.

[sssoleileraaa]

If you're not convinced, you can easily test whether or not your changes fix this issue by logging out when there are items left in the queue.

(I'm not entirely convinced ;))

[rmol]

lgtm