configure.sh: explicit error msg at failure #1496

Job	Run time
macOS--dotnet6-and-mono	10m 50s
macOS--mono	8m 9s
linux-oldLTS-github--dotnet-and-msbuild	7m 13s
linux-newLTS-github--dotnet-and-msbuild	8m 50s
linux-newLTS-github--dotnet-and-xbuild	7m 46s
linux-newLTS-vanilla--stockmono	6m 18s
linux-oldLTS-github--dotnet-and-xbuild	7m 45s
linux-oldLTS-vanilla--newmono	8m 4s
linux-oldLTS-vanilla--stockmono	6m 13s
linux-newLTS-vanilla--dotnet6	9m 31s
linux-newLTS-vanilla--newmono	7m 10s
windows--dotnet6	10m 33s
windows--legacyFramework	15m 40s
sanity-check	1m 18s
publish	1m 14s
	1h 56m 34s

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