only log export-no-custom-routes warning if unsupported

[ztanner]

This doesn't need to error, we can instead warn that the functionality will not work as expected out of the box. Support can be added for outside of Next for this to behave as expected.

These are supported when deployed via the Nextjs builder (x-ref).

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
buildDuration	10.8s	10.7s	N/A
buildDurationCached	6.2s	6.2s	N/A
nodeModulesSize	177 MB	178 MB	⚠️ +922 B
nextStartRea..uration (ms)	426ms	419ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
199-HASH.js gzip	27.9 kB	27.9 kB	N/A
3f784ff6-HASH.js gzip	53.2 kB	53.2 kB	N/A
99.HASH.js gzip	182 B	182 B	✓
framework-HASH.js gzip	45.5 kB	45.5 kB	✓
main-app-HASH.js gzip	254 B	251 B	N/A
main-HASH.js gzip	33 kB	33 kB	N/A
webpack-HASH.js gzip	1.75 kB	1.75 kB	N/A
Overall change	45.7 kB	45.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
_app-HASH.js gzip	206 B	205 B	N/A
_error-HASH.js gzip	182 B	180 B	N/A
amp-HASH.js gzip	506 B	505 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.59 kB	2.59 kB	✓
edge-ssr-HASH.js gzip	260 B	259 B	N/A
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	369 B	369 B	✓
image-HASH.js gzip	4.38 kB	4.38 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	316 B	318 B	N/A
script-HASH.js gzip	385 B	384 B	N/A
withRouter-HASH.js gzip	319 B	319 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.99 kB	3.99 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
_buildManifest.js gzip	484 B	482 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
index.html gzip	528 B	529 B	N/A
link.html gzip	542 B	541 B	N/A
withRouter.html gzip	525 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
edge-ssr.js gzip	95.3 kB	95.3 kB	N/A
page.js gzip	157 kB	157 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js fix/server-config-errors	Change
middleware-b..fest.js gzip	625 B	625 B	✓
middleware-r..fest.js gzip	150 B	151 B	N/A
middleware.js gzip	22.9 kB	22.9 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	2.55 kB	2.55 kB	✓

Commit: d3ed66a

[vercel-spaces]

Notifying the following users due to files changed in this PR based on this repo's notify modifiers:

@timneutkens, @ijjk, @shuding, @styfle, @huozhi:

packages/next/src/server/config.ts

[ijjk]

Tests Passed