bpo-38332: Catch KeyError from unknown cte in encoded-word. (GH-16503)

KeyError should cause a failure in parsing the encoded word and should be caught and raised as a _InvalidEWError instead.

Lib/email/_header_value_parser.py

@@ -1057,7 +1057,7 @@ def get_encoded_word(value):
     value = ''.join(remainder)
     try:
         text, charset, lang, defects = _ew.decode('=?' + tok + '?=')
-    except ValueError:
+    except (ValueError, KeyError):
         raise _InvalidEwError(
             "encoded word format invalid: '{}'".format(ew.cte))
     ew.charset = charset