ERR: Raise a simpler backtrace for missing key (#21558)

Author: toobaz

doc/source/whatsnew/v0.24.0.txt

@@ -197,7 +197,7 @@ Strings
 Indexing
 ^^^^^^^^
 
--
+- The traceback from a ``KeyError`` when asking ``.loc`` for a single missing label is now shorter and more clear (:issue:`21557`)
 -
 -
 

pandas/core/indexing.py

@@ -1807,8 +1807,6 @@ def error():
 
             try:
                 key = self._convert_scalar_indexer(key, axis)
-                if not ax.contains(key):
-                    error()
             except TypeError as e:
 
                 # python 3 type errors should be raised
@@ -1818,6 +1816,9 @@ def error():
             except:
                 error()
 
+            if not ax.contains(key):
+                error()
+
     def _is_scalar_access(self, key):
         # this is a shortcut accessor to both .loc and .iloc
         # that provide the equivalent access of .at and .iat