website: Add example for Elvish's command failure behavior to homepage. #1670

Job	Run time
Run benchmarks	1m 12s
Check relative links in website	15s
Run checks	46s
Upload SourceGraph LSIF	29s
Run tests (ubuntu, 1.22.x)	1m 28s
Run tests (macos, 1.22.x)	1m 18s
Run tests (windows, 1.22.x)	2m 23s
Run tests (ubuntu, 1.21.x)	41s
Upload test coverage (linux)	5s
Upload test coverage (macos)	10s
Upload test coverage (windows)	10s
	8m 57s

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