Better hash format error message (#1442)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_server/services/contents/handlers.py

@@ -140,7 +140,9 @@ async def get(self, path=""):
 
         hash_str = self.get_query_argument("hash", default="0")
         if hash_str not in {"0", "1"}:
-            raise web.HTTPError(400, f"Content {hash_str!r} is invalid")
+            raise web.HTTPError(
+                400, f"Hash argument {hash_str!r} is invalid. It must be '0' or '1'."
+            )
         require_hash = int(hash_str)
 
         if not cm.allow_hidden and await ensure_async(cm.is_hidden(path)):