Cannot change an aliased rule anymore: "rule with alias exists"

[JohanLorenzo]

Discovered by @wsmwk (cc @jfx2006)

Steps to reproduce

1. Change background rate of a rule (e.g.: https://balrog.services.mozilla.com/rules/43)
2. Submit changes

Expected result

New scheduled change is waiting for sign-offs

Actual result

UI shows error rule with alias exists

The backend returns a 400:

Warning ⚠️

I worked around the issue by modifying the alias of the rule alongside the background rate. Now the alias is thunderbird-beta1. I wanted to change it back but I get a second error: Rule is scheduled with the given alias

Additional notes

It's likely a regression from #3010 (#2942). It seems no newly added error message works as expected. I believe we should likely back it out and redeploy balrog. I didn't to it for 3 reasons:

1. The problem was worked around.
2. I'm the only Release Engineer today. Moreover, yesterday, I noticed I don't have access to the Jenkins instance anymore. I'm waiting for SRE to get this resolved.
3. It's a patch made by an external contributor.

@bhearsum @gabrielBusta, let me know what the best approach is to you.

[JohanLorenzo]

Firefox[1] and DevEdition[2] 120.0b9 were both impacted too. I worked around them with the same trick. I'm going to let the downstream tasks force-run by cancelling and rerunning them.

[1] https://balrog.services.mozilla.com/rules?product=Firefox&channel=beta#scId=5467
[2] https://balrog.services.mozilla.com/rules?product=Firefox&channel=aurora#scId=5468

[JohanLorenzo]

Bug 1864286 was also put on file.