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.
Added type annotations to function parameters, function return
values and local variables. Added neccessary `if` statements to
enforce the objects having proper type before executing the later
operations.
An error type check in the catch block in `update_favicon`
function is added because the statement in the catch block requires
error to be a type `Error` in order to access `error.stack`.