Eugene Yokota 06ffb4f440 warn about multiple instance once ...

Fixes #3823

When you launch a second instance of sbt on a build, prior to this change it was displaying `java.io.IOException: sbt server is already running` on every command. This make it a bit less aggressive, and just display a warning once.

```
[warn] Is another instance of sbt is running on this build?
[warn] Running multiple instances is unsupported
```

2017-12-20 10:09:03 -05:00

..

main

warn about multiple instance once

2017-12-20 10:09:03 -05:00