[llvm-pdbutil] Clean up ExitOnError usage to add ": " to our errors

The banner parameter is supposed to end in a separator, like ": ".
Otherwise, we get ugly errors like:

Error while reading publics streamNative error: blah blah

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309332 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed