Dotty 0.4.0-RC1 shell scripts should detect Java 9 and stop with an error

[smarter]

Dotty won't be compatible with Java 9 until #3138 is done. To avoid confusing people with weird errors, we should check the Java version in the shell script and ask people to use Java 8. To get the Java version use https://github.com/sbt/sbt-launcher-package/blob/master/src/universal/bin/sbt-launch-lib.bash

[smarter]

The actual check is at https://github.com/sbt/sbt-launcher-package/blob/master/src/universal/bin/sbt-launch-lib.bash#L183