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Simplejson already have JSONDecodeError.
class JSONDecodeError(ValueError):
 """Subclass of ValueError with the following additional properties:
 msg: The unformatted error message
 doc: The JSON document being parsed
 pos: The start index of doc where parsing failed
 end: The end index of doc where parsing failed (may be None)
 lineno: The line corresponding to pos
 colno: The column corresponding to pos
 endlineno: The line corresponding to end (may be None)
 endcolno: The column corresponding to end (may be None)
 """
Adding it to the stdlib is only matter of time.