Allow config get to work when db is not functional

Summary: Fixes T6554.

Test Plan:
Run `bin/config get mysql.host` when no db is functional. Should not get an exception, but should see:
```
lang=json
{
  "config" : [
    {
      "key"       : "mysql.host",
      "source"    : "local",
      "value"     : null,
      "status"    : "unset",
      "errorInfo" : null
    },
    {
      "key"       : "mysql.host",
      "source"    : "database",
      "value"     : null,
      "status"    : "error",
      "errorInfo" : "Database source is not configured properly"
    }
  ]
}
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T6554

Differential Revision: https://secure.phabricator.com/D10851