azure: return an early error if unaligned

If the image size isn't aligned to 512 bytes, the Azure API returns very hard
to understand error message. Let's do this check ourselves early so we can
return a sane error.

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

internal/upload/azure/azure.go

@@ -71,6 +71,10 @@ func UploadImage(credentials Credentials, metadata ImageMetadata, fileName strin
 		return fmt.Errorf("cannot stat the image: %v", err)
 	}
 
+	if stat.Size()%512 != 0 {
+		return errors.New("size for azure image must be aligned to 512 bytes")
+	}
+
 	// Hash the imageFile
 	imageFileHash := md5.New()
 	if _, err := io.Copy(imageFileHash, imageFile); err != nil {