file-restore: add context to b64-decode error

to make the following cryptic error:

 proxmox-file-restore failed: Error: Invalid byte 46, offset 5.

more understandable:

 proxmox-file-restore failed: Error: Failed base64-decoding path '/root.pxar.didx' - Invalid byte 46, offset 5.

when a user passes in a non-base64 path but sets `--base64`.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/bin/proxmox-file-restore.rs

@@ -47,7 +47,8 @@ enum ExtractPath {
 
 fn parse_path(path: String, base64: bool) -> Result<ExtractPath, Error> {
     let mut bytes = if base64 {
-        base64::decode(path)?
+        base64::decode(path.clone())
+            .map_err(|err| format_err!("Failed base64-decoding path '{}' - {}", path, err))?
     } else {
         path.into_bytes()
     };