Instead of propagating the errors as a `bool`+`Stream` we change the
`ClangModulesDeclVendor` module loading APIs to use `llvm::Error`. We
also reword some of the diagnostics (notably removing the hardcoded
`error:` prefix). A follow-up patch will further make the module loading
errors less noisy.
See the new tests for what the errors look like.
rdar://164002569
