Emit warning if no dist is available

[andersevenrud]

This came up during a conversation on Gitter.

It would probably be useful if the server could check the dist/ directory, and if it was not satisfied a message would appear with some suggestions of what might have gone wrong:

• Forgot to run npm run buld ?
• Did an error occur while running npm run buld ?

[RossComputerGuy]

So like a log file that the server checks to see if dist was correctly created?

[andersevenrud]

Just print to stderr if index.html or the pattern osjs.* was not found inside dist/ .

[RossComputerGuy]

Ah