Error gracefully for unsupported SSR features

[gaearon]

While we may plan to make Suspense work for some cases for the current SSR (I think?), in the meantime I'm adding coverage for the current behavior and replacing highly unintuitive Invalid element type errors that happen today.

I also added a test for client-only post-hydration use of Suspense, to avoid accidentally regressing this.

[sizebot]

Details of bundled changes.

Comparing: 6d5d250...821a421

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom-server.browser.development.js	+0.2%	+0.2%	106.3 KB	106.56 KB	28.25 KB	28.3 KB	UMD_DEV
react-dom-server.browser.production.min.js	🔺+0.4%	🔺+0.4%	15.47 KB	15.53 KB	5.92 KB	5.95 KB	UMD_PROD
react-dom-server.browser.development.js	+0.3%	+0.2%	102.43 KB	102.69 KB	27.25 KB	27.3 KB	NODE_DEV
react-dom-server.browser.production.min.js	🔺+0.4%	🔺+0.3%	15.38 KB	15.44 KB	5.86 KB	5.88 KB	NODE_PROD
react-dom-server.node.development.js	+0.3%	+0.2%	104.35 KB	104.61 KB	27.81 KB	27.86 KB	NODE_DEV
react-dom-server.node.production.min.js	🔺+0.4%	🔺+0.3%	16.18 KB	16.24 KB	6.16 KB	6.18 KB	NODE_PROD
ReactDOMServer-dev.js	+0.3%	+0.2%	101.85 KB	102.16 KB	26.53 KB	26.59 KB	FB_WWW_DEV
ReactDOMServer-prod.js	🔺+0.4%	🔺+0.3%	33.87 KB	34 KB	8.15 KB	8.17 KB	FB_WWW_PROD

scheduler

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
scheduler.development.js	n/a	n/a	0 B	19.17 KB	0 B	5.74 KB	UMD_DEV
scheduler.production.min.js	n/a	n/a	0 B	3.16 KB	0 B	1.53 KB	UMD_PROD

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[trueadm]

LGTM