[cmake] Switch FATAL_ERROR to SEND_ERROR

It's possible for multiple distribution components to have missing
targets, and it's a lot more convenient to get all those errors in one
shot rather than having to fix them individually.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317148 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed