sentry: refactor initialization sequence
sentry: refactor initialization sequence

... mostly to add comments; but also to add a `console.log`, to
confirm that Sentry initialization has taken place. No functional
changes otherwise.

As one of the comments notes, calling Sentry functions without
initializing Sentry "has worked [...] so far". This is hardly
reassuring, but will be bolstered by tests in an upcoming commit.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile