SI-6158 Restore compile error output under partest --show-log

Seems like the ifs and elses didn't quite survive e830a7ce9.

Before:

    ./test/partest --show-log test/files/run/foo.scala
    Testing individual files
    testing: [...]/files/run/foo.scala                                    [FAILED]

Now:

    ./test/partest --show-log test/files/run/foo.scala
    Testing individual files
    testing: [...]/files/run/foo.scala                                    [FAILED]
    foo.scala:1: error: expected class or object definition
    askdfjskl
    ^
    one error found
    1 of 1 tests failed (elapsed time: 00:00:01)

Review by @VladUreche

The part I'm not 100% sure about is the duplicated output below: we show both a diff and the full output. I could also silence the full output if the diff is non empty; suggestions here are welcome.

    ./test/partest --show-log  test/files/run/foo.scala
    
    Testing individual files
    testing: [...]/files/run/foo.scala                                    [FAILED]
    1c1
    < 11
    ---
    > 1
    11