Improve TestUtils error messages for bad first argument

[gaearon]

This makes errors like #11855 a bit clearer.
Don't think we'll want to support passing DOM nodes as it would require relying on more internals.

[pull-bot]

Details of bundled changes.

Comparing: d59b993...9711aa9

react-dom

File	Filesize Diff	Gzip Diff	Prev Size	Current Size	Prev Gzip	Current Gzip	ENV
react-dom-test-utils.development.js	+2.3%	+1.9%	43.74 KB	44.76 KB	11.9 KB	12.12 KB	UMD_DEV
react-dom-test-utils.production.min.js	🔺+4.2%	🔺+3.8%	10.06 KB	10.48 KB	3.72 KB	3.86 KB	UMD_PROD
react-dom-test-utils.development.js	+2.3%	+1.9%	43.46 KB	44.48 KB	11.84 KB	12.06 KB	NODE_DEV
react-dom-test-utils.production.min.js	🔺+4.3%	🔺+3.4%	9.85 KB	10.28 KB	3.66 KB	3.79 KB	NODE_PROD
ReactTestUtils-dev.js	+2.5%	+2.1%	40.66 KB	41.66 KB	10.92 KB	11.15 KB	FB_WWW_DEV

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