Improve the "failed to download" error message

Example:
```yaml
images:
- location: "https://example.com/dummy-x86_64"
  arch: "x86_64"
- location: "https://example.com/dummy-aarch64"
  arch: "aarch64"
```

Before:
```console
$ limactl start dummy
INFO[0000] Using the existing instance "dummy"
INFO[0000] Attempting to download the image              arch=x86_64 digest= location="https://example.com/dummy-x86_64"
FATA[0000] failed to download the image, attempted 2 candidates, errors=[failed to download "https://example.com/dummy-x86_64": expected HTTP status 200, got 404 Not Found unsupported arch: "aarch64"]
```

After:
```console
$ limactl start dummy
INFO[0000] Using the existing instance "dummy"
INFO[0000] Attempting to download the image              arch=x86_64 digest= location="https://example.com/dummy-x86_64"
FATA[0000] failed to download "https://example.com/dummy-x86_64": expected HTTP status 200, got 404 Not Found
```

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

Author: AkihiroSuda

pkg/fileutils/download.go

@@ -1,6 +1,7 @@
 package fileutils
 
 import (
+	"errors"
 	"fmt"
 	"path"
 
@@ -9,10 +10,13 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
+// ErrSkipped is returned when the downloader did not attempt to download the specified file.
+var ErrSkipped = errors.New("skipped to download")
+
 // DownloadFile downloads a file to the cache, optionally copying it to the destination. Returns path in cache.
 func DownloadFile(dest string, f limayaml.File, decompress bool, description string, expectedArch limayaml.Arch) (string, error) {
 	if f.Arch != expectedArch {
-		return "", fmt.Errorf("unsupported arch: %q", f.Arch)
+		return "", fmt.Errorf("%w: %q: unsupported arch: %q", ErrSkipped, f.Location, f.Arch)
 	}
 	fields := logrus.Fields{"location": f.Location, "arch": f.Arch, "digest": f.Digest}
 	logrus.WithFields(fields).Infof("Attempting to download %s", description)
@@ -36,3 +40,25 @@ func DownloadFile(dest string, f limayaml.File, decompress bool, description str
 	}
 	return res.CachePath, nil
 }
+
+// Errors compose multiple into a single error.
+// Errors filters out ErrSkipped.
+func Errors(errs []error) error {
+	var finalErr error
+	for _, err := range errs {
+		if errors.Is(err, ErrSkipped) {
+			logrus.Debug(err)
+		} else {
+			if finalErr == nil {
+				finalErr = err
+			} else {
+				finalErr = fmt.Errorf("%v, %w", finalErr, err)
+			}
+		}
+	}
+	if len(errs) > 0 && finalErr == nil {
+		// errs only contains ErrSkipped
+		finalErr = fmt.Errorf("%v", errs)
+	}
+	return finalErr
+}

pkg/qemu/qemu.go

@@ -76,8 +76,7 @@ func EnsureDisk(cfg Config) error {
 			break
 		}
 		if !ensuredBaseDisk {
-			return fmt.Errorf("failed to download the image, attempted %d candidates, errors=%v",
-				len(cfg.LimaYAML.Images), errs)
+			return fileutils.Errors(errs)
 		}
 	}
 	diskSize, _ := units.RAMInBytes(*cfg.LimaYAML.Disk)

pkg/start/start.go

@@ -53,8 +53,7 @@ func ensureNerdctlArchiveCache(y *limayaml.LimaYAML) (string, error) {
 		return path, nil
 	}
 
-	return "", fmt.Errorf("failed to download the nerdctl archive, attempted %d candidates, errors=%v",
-		len(y.Containerd.Archives), errs)
+	return "", fileutils.Errors(errs)
 }
 
 func Start(ctx context.Context, inst *store.Instance) error {

pkg/vz/disk.go

@@ -37,8 +37,7 @@ func EnsureDisk(driver *driver.BaseDriver) error {
 			break
 		}
 		if !ensuredBaseDisk {
-			return fmt.Errorf("failed to download the image, attempted %d candidates, errors=%v",
-				len(driver.Yaml.Images), errs)
+			return fileutils.Errors(errs)
 		}
 	}
 	diskSize, _ := units.RAMInBytes(*driver.Yaml.Disk)