Fix double-formatting of UserExceptions
Fix double-formatting of UserExceptions

Previously, both the __str__ method of UserException would run the
exception through WalELogger.fmt_logline() and so would the
instance-method to WalELogger.log().  This resulted in output like:

    wal_e.main   ERROR    MSG: MSG: invalid backup query submitted
            DETAIL: The submitted query operator was "/tmp."
            STRUCTURED: time=2013-09-08T11:28:32.788848-00 pid=2130
            STRUCTURED: time=2013-09-08T11:28:32.788912-00 pid=2130

Where "MSG" appears twice and "STRUCTURED" appears twice.

The second level of formatting treated the first level as an opaque
string, which is why only "MSG" and "STRUCTURED" were repeated.

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