fix: str of KeyError for <3.11 (#2519)

Co-authored-by: Jim Pivarski <[email protected]>

src/awkward/_errors.py

@@ -1,6 +1,7 @@
 # BSD 3-Clause License; see https://github.com/scikit-hep/awkward-1.0/blob/main/LICENSE
 from __future__ import annotations
 
+import builtins
 import sys
 import threading
 import warnings
@@ -31,6 +32,11 @@ def __call__(self):
         return self.func(*self.args, **self.kwargs)
 
 
+class KeyError(builtins.KeyError):
+    def __str__(self):
+        return super(Exception, self).__str__()
+
+
 class ErrorContext:
     # Any other threads should get a completely independent _slate.
     _slate = threading.local()
@@ -87,6 +93,9 @@ def decorate_exception(self, cls: type[E], exception: E) -> E:
                     + "\n\nSee if this has been reported at https://github.com/scikit-hep/awkward/issues"
                 )
                 new_exception.__cause__ = exception
+            elif issubclass(cls, builtins.KeyError):
+                new_exception = KeyError(self.format_exception(exception))
+                new_exception.__cause__ = exception
             else:
                 new_exception = cls(self.format_exception(exception))
                 new_exception.__cause__ = exception