Merge pull request juju#17929 from hmlanigan/update-juju-client-backu…

…p-download

juju#17929

Rather than a 404 message, print a nice error message if attempting to download a backup archive from a 4.0 controller, or any other which does not support the action.

## Checklist

- [ ] Code style: imports ordered, good names, simple structure, etc
- [ ] Comments saying why design decisions were made
- [ ] Go unit tests, with comments saying what you're testing
- [ ] [Integration tests](https://github.com/juju/juju/tree/main/tests), with comments saying what you're testing
- [ ] [doc.go](https://discourse.charmhub.io/t/readme-in-packages/451) added or updated in changed packages

## QA steps

If it hasn't been merged into main yet, use juju#17928 to bootstrap juju.

```
# Bootstrap juju from juju#17928 unless it's already merged. In that case use juju 4.0.
$ juju bootstrap lxd no-backup-download

# using the juju client built from this PR
$ juju download-backup -m controller /tmp/testme
Download of backup archive files is not supported by this controller.
```

## Links

**Jira card:** JUJU-6470

api/client/backups/download.go

@@ -10,6 +10,7 @@ import (
 	"github.com/juju/errors"
 	"gopkg.in/httprequest.v1"
 
+	apiservererrors "github.com/juju/juju/apiserver/errors"
 	"github.com/juju/juju/rpc/params"
 )
 
@@ -36,7 +37,7 @@ func (c *Client) Download(filename string) (io.ReadCloser, error) {
 		&resp,
 	)
 	if err != nil {
-		return nil, errors.Trace(err)
+		return nil, errors.Trace(apiservererrors.RestoreError(err))
 	}
 	return resp.Body, nil
 }

cmd/juju/backups/download.go

@@ -88,17 +88,21 @@ func (c *downloadCommand) Run(ctx *cmd.Context) error {
 	// Download the archive.
 	resultArchive, err := client.Download(c.RemoteFilename)
 	if err != nil {
+		if errors.Is(err, errors.NotFound) {
+			ctx.Errorf("Download of backup archive files is not supported by this controller.")
+			return nil
+		}
 		return errors.Trace(err)
 	}
-	defer resultArchive.Close()
+	defer func() { _ = resultArchive.Close() }()
 
 	// Prepare the local archive.
 	filename := c.ResolveFilename()
 	archive, err := c.Filesystem().Create(filename)
 	if err != nil {
 		return errors.Annotate(err, "while creating local archive file")
 	}
-	defer archive.Close()
+	defer func() { _ = archive.Close() }()
 
 	// Write out the archive.
 	_, err = io.Copy(archive, resultArchive)