Use _error for development in streaming

[huozhi]

Follow up for #31164

• pathname should be _error for correct hydration
• use _error in development instead of trying to require the custom 500 page (Avoid module not found error for /500)

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
buildDuration	21.1s	21s	-61ms
buildDurationCached	4.2s	4.1s	-37ms
nodeModulesSize	336 MB	336 MB	-56 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.42	3.467	⚠️ +0.05
/ avg req/sec	730.99	721.12	⚠️ -9.87
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.669	1.561	-0.11
/error-in-render avg req/sec	1497.87	1601.85	+103.98

Client Bundles (main, webpack, commons)

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28.3 kB	28.3 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	72.2 kB	72.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
_app-HASH.js gzip	1.37 kB	1.37 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.44 kB	4.44 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.3 kB	13.3 kB	✓

Client Build Manifests

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
index.html gzip	523 B	523 B	✓
link.html gzip	536 B	536 B	✓
withRouter.html gzip	517 B	517 B	✓
Overall change	1.58 kB	1.58 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
buildDuration	22.5s	22.8s	⚠️ +279ms
buildDurationCached	4.1s	4.1s	⚠️ +12ms
nodeModulesSize	336 MB	336 MB	-56 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.458	3.431	-0.03
/ avg req/sec	723.03	728.75	+5.72
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.62	1.606	-0.01
/error-in-render avg req/sec	1543.66	1556.51	+12.85

Client Bundles (main, webpack, commons)

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.5 kB	28.5 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	72.5 kB	72.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.46 kB	4.46 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.2 kB	13.2 kB	✓

Client Build Manifests

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	huozhi/next.js fix/streaming-500	Change
index.html gzip	524 B	524 B	✓
link.html gzip	535 B	535 B	✓
withRouter.html gzip	517 B	517 B	✓
Overall change	1.58 kB	1.58 kB	✓

Commit: d7bd785