SI-7781 REPL stack trunc shows cause

The handy stack trace truncation in REPL doesn't
show cause like a regular trace.

This commit fixes that and also adds the usual
indicator for truncation, viz, "... 33 more".

The example from the ticket produces:

```
scala> rewrapperer
java.lang.RuntimeException: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .rewrapper(<console>:9)
  at .rewrapperer(<console>:10)
  ... 32 elided
Caused by: java.lang.RuntimeException: java.lang.RuntimeException: Point of failure
  at .wrapper(<console>:8)
  ... 34 more
Caused by: java.lang.RuntimeException: Point of failure
  at .sample(<console>:7)
  ... 35 more
```

Suppressed exceptions on Java 7 are handled reflectively.

```
java.lang.RuntimeException: My problem
  at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:56)
  ... 27 elided
  Suppressed: java.lang.RuntimeException: Point of failure
    at scala.tools.nsc.util.StackTraceTest.sample(StackTraceTest.scala:29)
    at scala.tools.nsc.util.StackTraceTest.repressed(StackTraceTest.scala:54)
    ... 27 more
```
Target junit.clean to clean junit artifacts

And all.clean will also junit.clean.