ref: Avoid using SentryError for PromiseBuffer control flow

[mydea]

This replaces usage of SentryError for promise buffer control flow.
Instead, we can use a symbol and just check this directly, we do not even care about the message here.

ref #15725 (comment)

[github-actions]

size-limit report 📦

Path	Size	% Change	Change
@sentry/browser	23.22 KB	+0.03%	+7 B 🔺
@sentry/browser - with treeshaking flags	23.02 KB	+0.03%	+6 B 🔺
@sentry/browser (incl. Tracing)	36.62 KB	+0.03%	+8 B 🔺
@sentry/browser (incl. Tracing, Replay)	73.79 KB	+0.01%	+3 B 🔺
@sentry/browser (incl. Tracing, Replay) - with treeshaking flags	67.13 KB	+0.03%	+14 B 🔺
@sentry/browser (incl. Tracing, Replay with Canvas)	78.42 KB	+0.01%	+7 B 🔺
@sentry/browser (incl. Tracing, Replay, Feedback)	91.01 KB	+0.01%	+7 B 🔺
@sentry/browser (incl. Feedback)	40.34 KB	+0.02%	+7 B 🔺
@sentry/browser (incl. sendFeedback)	27.85 KB	+0.02%	+5 B 🔺
@sentry/browser (incl. FeedbackAsync)	32.64 KB	+0.02%	+5 B 🔺
@sentry/react	25 KB	+0.02%	+4 B 🔺
@sentry/react (incl. Tracing)	38.52 KB	+0.04%	+15 B 🔺
@sentry/vue	27.44 KB	+0.03%	+6 B 🔺
@sentry/vue (incl. Tracing)	38.31 KB	+0.04%	+13 B 🔺
@sentry/svelte	23.25 KB	+0.03%	+7 B 🔺
CDN Bundle	24.44 KB	+0.03%	+6 B 🔺
CDN Bundle (incl. Tracing)	36.63 KB	-0.01%	-1 B 🔽
CDN Bundle (incl. Tracing, Replay)	71.63 KB	+0.03%	+15 B 🔺
CDN Bundle (incl. Tracing, Replay, Feedback)	76.83 KB	+0.01%	+4 B 🔺
CDN Bundle - uncompressed	71.36 KB	-0.05%	-30 B 🔽
CDN Bundle (incl. Tracing) - uncompressed	108.55 KB	-0.03%	-30 B 🔽
CDN Bundle (incl. Tracing, Replay) - uncompressed	219.8 KB	-0.02%	-30 B 🔽
CDN Bundle (incl. Tracing, Replay, Feedback) - uncompressed	232.37 KB	-0.02%	-30 B 🔽
@sentry/nextjs (client)	39.81 KB	+0.02%	+7 B 🔺
@sentry/sveltekit (client)	37.04 KB	+0.03%	+10 B 🔺
@sentry/node	142.85 KB	-0.01%	-1 B 🔽
@sentry/node - without tracing	96.24 KB	-0.01%	-6 B 🔽
@sentry/aws-serverless	120.59 KB	+0.01%	+1 B 🔺

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