IDE does not start if the project does not compile

[nicolasstucki]

When launching the IDE with sbt launchIDE and the project sources do not compile, the IDE simply does not start.

This can be reproduced by creating a new project with sbt new lampepfl/dotty.g8, adding some code that does not compile in the main class and sbt launchIDE.

[smarter]

This is intentional, the IDE won't work right if up-to-date classfiles are not present. The alternative is loading all source files with out-of-date or non-existent classfiles in the IDE, but this requires communication with the build tool, which is hard to do until sbt gets a working server mode.

[Blaisorblade]

FWIW in #4469 @smarter has a (seemingly different) suggestion here.

[Duhemm]

@smarter improved Dotty IDE so that it starts with the old configuration if the projects doesn't compile here: #5128