Add v:lua.func() vimL syntax for calling lua

Also simplify error messages when calling lua from vimL.

src/nvim/eval.h

@@ -117,6 +117,7 @@ typedef enum {
     VV_TYPE_BOOL,
     VV_ECHOSPACE,
     VV_EXITING,
+    VV_LUA,
 } VimVarIndex;
 
 /// All recognized msgpack types