Notice localhost mistake?

I hit on the same issue as https://community.fly.io/t/cant-make-fly-pg-import-to-work/21577/2 because I'm used to doing `fly proxy` and putting in localhost as the hostname, while the importer seems to create a worker machine and so needs to use the "real" host. That's all well and fine, but it could save someone a bit of head-scratching if the error message was clearer, currently it says

```
$ flyctl postgres import --app my-app-db "$DATABASE_URL"
Some regions require a Launch plan or higher (bom).
See https://fly.io/plans to set up a plan.

? Choose a region to deploy the migration machine: Amsterdam, Netherlands (ams)
? Select VM size: shared-cpu-1x - CPU Kind: Shared, vCPUs: 1 Memory: 256MB
Created an ephemeral machine 48ed439fd57408 to run the import process.
Connecting to fdaa:1:6dd4:a7b:13b:cc11:a9fc:2... complete
[info] Running pre-checks...
Waiting for ephemeral machine 48ed439fd57408 to be destroyed ... done.
Error: failed to run ssh: ssh shell: Process exited with status 1
```

which isn't very helpful. One option would be for `--help` to note that the `` has to be reachable from fly.io so don't use localhost proxy, alternatively maybe before the failing ssh shell step run a little check for reachability which could fail with "not able to reach source database host"