Give better error message when fixture project is missing a file #15876

Job	Run time
Lint	1m 30s
ci (bundler, bundler1, 2, 0, bundler)	4m 7s
ci (bundler, bundler1, 2, 1, bundler)	4m 11s
ci (bundler, bundler2, 2, 0, bundler)	4m 18s
ci (bundler, bundler2, 2, 1, bundler)	3m 21s
ci (cargo, cargo, cargo)	6s
ci (common, common, common)	2m 4s
ci (composer, composer, 2, 0, composer)	7s
ci (composer, composer, 2, 1, composer)	5s
ci (docker, docker, docker)	4s
ci (elm, elm, elm)	6s
ci (git_submodules, git_submodules, gitsubmodule)	7s
ci (github_actions, github_actions, github-actions)	7s
ci (go_modules, go_module, 2, 0, gomod)	4s
ci (go_modules, go_module, 2, 1, gomod)	7s
ci (gradle, gradle, gradle)	4s
ci (hex, hex, 2, 0, mix)	4s
ci (hex, hex, 2, 1, mix)	6s
ci (maven, maven, maven)	7s
ci (npm_and_yarn, npm_and_yarn, 3, 0, npm)	4s
ci (npm_and_yarn, npm_and_yarn, 3, 1, npm)	5s
ci (npm_and_yarn, npm_and_yarn, 3, 2, npm)	6s
ci (nuget, nuget, nuget)	6s
ci (pub, pub, 2, 0, pub)	6s
ci (pub, pub, 2, 1, pub)	7s
ci (python, python, 2, 0, pip)	4s
ci (python, python, 2, 1, pip)	5s
ci (python, python_slow, 2, 0, pip)	4s
ci (python, python_slow, 2, 1, pip)	4s
ci (swift, swift, 2, 0, swift)	7s
ci (swift, swift, 2, 1, swift)	4s
ci (terraform, terraform, terraform)	6s
Updater	3m 7s
	25m 0s

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