Refuse to start or issue a warning if microraiden.js is not available

[pcppcp]

With the new typescript library the microraiden.js must be compiled manually. We may want to check for its existence when starting the proxy and notify the user if it is missing.

The warning may be displayed both in the webui and in the proxy log.

[andrevmatos]

IMO, a warning in uppercase in the logs should be enough, instead of refusing to start, as it can be generated and will be available directly, no restart required. Also, it can be proxyed by e.g. nginx, so it may be available to the client but not in the usual folder. Don't think it should be done in the webui, as the webui doesn't start properly if there's no microraiden.js.