postcompile/precompile doesn't respect exit code of the command

[krwq]

compile doesn't fail when the postcompile/pre command failed.

[TheRealPiotrP]

We expect to be dropping script support, so putting this off for now.

[analogrelay]

Are we still dropping script support? /cc @piotrpMSFT @davidfowl

If not, this is affecting our own builds because we do a dotnet build in a precompile script (for ArgumentForwarderTests) and if it fails, the build doesn't fail.

[TheRealPiotrP]

The team is actively working on enabling MSBuild and the component affected will be superseded by the new project system, so I am closing this issue.