This removes the production reporting to `/json/report/error` upon
`blueslip.error`, and replaces it with reporting to Sentry, if
enabled. Sentry provides better reporting and grouping for exceptions
than the email- and `#errors`-reporting provided by the
`/json/report/error` endpoint.
The development behaviour of rendering `blueslip.error` messages and
stacktraces immediately, and stopping execution, is preserved.
To better chain exception information, the whole previous exception is
passed to `blueslip.error`, not just the stack, and the second
parameter is formalized to be an object to map to Sentry's "context"
concept.
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