I think its cleaner if we just exit with an error if emscripten is not
yet configured.  Especially since we already have an error message for
"config file not found" so this change means that first time users will
now see that message (in which we now recommend the use of
`--generate-config`).

There are couple of reasons why I think this is a good idea:

1. We already have a error message for missing config file
2. Currently, when the config file is generated we return 0 without
   actually doing anything, which will confuse build systems.
3. In the case of a misconfigured system (e.g. an accidentally
   removed config file, or a EM_CONFIG pointing the wrong place),
   the solution is probably not to simply just create a new
   config, but to inform the user and let them take action.

Fixes: #13962