Improve the error message for when a build's main class is ambiguous

[Gedochao]

Fixes #1315

Before:

$ scala-cli run . [error] Found several main classes: ScalaMainClass2, ScalaMainClass1, scripts.ScalaScript_sc 

After these changes:

$ scala-cli run . [error] Found several main classes: ScalaMainClass1, ScalaMainClass2, scripts.ScalaScript_sc You can run one of them by passing it with the --main-class option, e.g. scala-cli . --main-class ScalaMainClass1 You can pick the main class interactively by passing the --interactive option. scala-cli . --interactive 

[alexarchambault]

Ok to merge, but just so you know, initially, for command suggestions such as these, my plan was to handle those like this one, that is catching specific exceptions in ScalaCli, and print suggestions for them.

That way, CLI-specific concerns don't leak in the build module. Originally, my intent was to maybe allow the build modules to be used as a library, where the CLI isn't around. So these suggestions don't make sense there. That said, I'm not sure the build modules are going to be used this way at some point, so it's no big deal...