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