fix(nextjs): Catch rejecting flushes

[lforst]

We missed catching flushes in a few places which could cause problems if there are issues when sending events.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser (incl. Tracing, Replay, Feedback) - Webpack (gzipped)	75.04 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack (gzipped)	66.38 KB (0%)
@sentry/browser (incl. Tracing, Replay) - Webpack with treeshaking flags (gzipped)	59.99 KB (0%)
@sentry/browser (incl. Tracing) - Webpack (gzipped)	31.13 KB (0%)
@sentry/browser (incl. Feedback) - Webpack (gzipped)	29.74 KB (0%)
@sentry/browser - Webpack (gzipped)	21.39 KB (0%)
@sentry/browser (incl. Tracing, Replay, Feedback) - ES6 CDN Bundle (gzipped)	71.61 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (gzipped)	63.33 KB (+0.02% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (gzipped)	29.67 KB (+0.03% 🔺)
@sentry/browser - ES6 CDN Bundle (gzipped)	21.72 KB (+0.05% 🔺)
@sentry/browser (incl. Tracing, Replay) - ES6 CDN Bundle (minified & uncompressed)	199.58 KB (+0.03% 🔺)
@sentry/browser (incl. Tracing) - ES6 CDN Bundle (minified & uncompressed)	89.94 KB (+0.06% 🔺)
@sentry/browser - ES6 CDN Bundle (minified & uncompressed)	64.92 KB (+0.08% 🔺)
@sentry/browser (incl. Tracing) - ES5 CDN Bundle (gzipped)	32.34 KB (+0.03% 🔺)
@sentry/react (incl. Tracing, Replay) - Webpack (gzipped)	66.75 KB (0%)
@sentry/react - Webpack (gzipped)	21.44 KB (0%)
@sentry/nextjs Client (incl. Tracing, Replay) - Webpack (gzipped)	83.48 KB (0%)
@sentry/nextjs Client - Webpack (gzipped)	48.23 KB (0%)
@sentry-internal/feedback - Webpack (gzipped)	16.19 KB (0%)