cleanup: reduce some duplicate code, avoid function pointers for a condition

Add 'multiline' flag to history for correct :messages output

Use larger buffer size for multiline messages. if this turns out to not
be enough, we could do size calculation like api_set_error

src/nvim/lua/executor.c

@@ -50,7 +50,7 @@ static void nlua_error(lua_State *const lstate, const char *const msg)
   size_t len;
   const char *const str = lua_tolstring(lstate, -1, &len);
 
-  emsgf_echo(msg, (int)len, str);
+  emsgf_multiline(msg, (int)len, str);
 
   lua_pop(lstate, 1);
 }