logging: Fix duplicate detection for email errors.
logging: Fix duplicate detection for email errors.

Our implementation of duplication detection in the Zulip email error
reporting system was buggy in two important ways:

* It did not look at the traceback, and thus considered all errors as
  the same.

* It reset the 10-minute duplicate timer every time an error happened,
  thus concealing situations where the same error was occuring more
  often than 1/10 minutes.

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