a8n: replacer service responds with 503 with cold cache

Created by: mrnugget

When previewing a campaign over a large number of repositories and the replacer cache in /tmp/replacer-archives is empty, replacer might respond with 503 Service Unavailable.

Example: with a fresh Sourcegraph instance, locally, I ran the following CampaignPlan:

{
    "scopeQuery": "repo:github",
    "matchTemplate": "foobar",
    "rewriteTemplate": "barfoo"
}

That resulted in 173 campaign jobs.

Out of those, 17 resulted in the mentioned error:

# SELECT error FrOM campaign_jobs WHERE error != '';
                                    error
-----------------------------------------------------------------------------
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"
 unexpected response status from replacer service: "503 Service Unavailable"