Name the S3 keys when storage_url moves to s3:// first (closes #428)

On a config that loads, `config set storage_url s3://...` is refused
until `s3.access_key_id` and `s3.secret_access_key` are set. The refusal
now says to set those two keys first, then `storage_url`.

Model: opus-5-5

internal/config/config.go

@@ -79,8 +79,13 @@ var (
 		"storage not configured; set storage_url or provide s3.endpoint + " +
 			"s3.bucket + credentials")
 	errS3BucketRequired = errors.New("s3.bucket is required (or set storage_url)")
-	errS3KeyIDRequired  = errors.New("s3.access_key_id is required")
-	errS3SecretRequired = errors.New("s3.secret_access_key is required")
+)
+
+// Validate returns these when S3 storage is configured without its
+// credentials.
+var (
+	ErrS3KeyIDRequired  = errors.New("s3.access_key_id is required")
+	ErrS3SecretRequired = errors.New("s3.secret_access_key is required")
 )
 
 // expandTilde expands ~ at the start of a path to the user's home directory.
@@ -522,11 +527,11 @@ func (c *Config) validateStorage() error {
 	}
 
 	if c.S3.AccessKeyID == "" {
-		return errS3KeyIDRequired
+		return ErrS3KeyIDRequired
 	}
 
 	if c.S3.SecretAccessKey == "" {
-		return errS3SecretRequired
+		return ErrS3SecretRequired
 	}
 
 	return nil
@@ -553,11 +558,11 @@ func (c *Config) validateStorageURL() error {
 	case strings.HasPrefix(c.StorageURL, "s3://"):
 		// S3 storage needs credentials
 		if c.S3.AccessKeyID == "" {
-			return fmt.Errorf("%w for s3:// URLs", errS3KeyIDRequired)
+			return fmt.Errorf("%w for s3:// URLs", ErrS3KeyIDRequired)
 		}
 
 		if c.S3.SecretAccessKey == "" {
-			return fmt.Errorf("%w for s3:// URLs", errS3SecretRequired)
+			return fmt.Errorf("%w for s3:// URLs", ErrS3SecretRequired)
 		}
 
 		return nil