When a project has failed imports, no targets should be run

See repro steps in https://github.com/dotnet/arcade/issues/2377

Historically, a failed import in the project file would prevent the project from loading. At some point CPS started skipping the imports and loading the project nonetheless. When in this state, running targets can have unwanted side effects. In the linked repro, output directories are not configured correctly due to failed imports and then the design time build causes intermediate files to be written all over the tree. The side effects could be anything else. Authors of msbuild logic cannot reasonably defend against an import they have having just been skipped. It should be considered incorrect/unsafe to run targets in this state.