Make the message associated with a fatal error slightly more helpful

Make the message associated with a fatal error slightly more helpful

Looking into 23095, my best guess is that the CodeGen library itself isn't getting linked and initialized properly.  To make this slightly more obvious to consumers of LLVM, emit a different error message if we can tell that the registry is empty vs you've simply happened to name a collector which hasn't been registered.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@235824 91177308-0d34-0410-b5e6-96231b3b80d8