Exit on main arg parse error

[som-snytt]

Could be a better way to construct the expr, there is Definitions.SystemModule.

Could use a test.

As a user, I never want to see stack trace from the arg mechanism. Parse error could take a cause, perhaps.

Fixes #16040

[som-snytt]

Mima body-slammed me on fiddling with ParseError, which is not internal, perhaps, so I'll retract that change.

But maybe I can get away with fixing

scala> val pe = util.CommandLineParser.ParseError(0, "uh oh")
val pe: scala.util.CommandLineParser.ParseError = scala.util.CommandLineParser$ParseError

scala> pe.getMessage
val res0: String = null

assuming that is an oversight.

[som-snytt]

This is a good one.

Test 'tests/run/Pouring.scala' failed with output:                              
Illegal command line: more arguments expected

The run test was given command line args which have always been missing, and now the test correctly fails on that basis.

[som-snytt]

I found out why they don't do that.

Referring to non-existent method static java.lang.System.exit(int)void
  called from static Test.main([java.lang.String)void
  called from core module module initializers
Test 'tests/run/quotes-add-erased' failed with output:                          
org.scalajs.linker.interface.LinkingException: There were linking errors

[som-snytt]

I will probably give up trying to accommodate ScalaJS.