push: treat all missing referenced files as fatal

`try_exists` will return Ok(false) if the path is or containts a dangling
symlink, treat that as hard error just like if `try_exists` has returned an
Err(..).

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

src/server/push.rs

@@ -849,7 +849,7 @@ pub(crate) async fn push_snapshot(
                 }
             }
         } else {
-            warn!("{path:?} does not exist, skipped.");
+            bail!("{path:?} does not exist, aborting upload.");
         }
     }