llvm-cov: Conformed headers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196541 91177308-0d34-0410-b5e6-96231b3b80d8
llvm-cov: Further improved error messages.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196542 91177308-0d34-0410-b5e6-96231b3b80d8