scala-cli export: Misleading error message when no dest

[bjornregnell]

Version(s)

$ scala-cli version
0.1.4

Describe the bug

When -o dest is omitted in scala-cli export the error message that comes after a looong time (see #957) is misleading, as it says dest already exists, perhaps because when omitting -o then scala-cli assumes current dir and that already exists...

To Reproduce

$ cat main.scala 
@main def run = println("hello")

$ time scala-cli export --sbt main.scala
Error: dest already exists.

Expected behaviour

Either 1) An error message saying that -o is missing or else perhaps 2) if no dest then assume a default dest like sbt-proj or something and report the name of the created dest.