separate std-out and std-err

I suggest that StarExec capture stdout and stderr from a solver separately - currently they are conflated.
In particular, SMTLIB prescribes the stdout output, but leaves the diagnostic output on stderr open.
By combining them, StarExec creates the possibility of confusing the postprocessor and reporting incorrect results.

I realize this change would have some ramifications - on storing job output, on displaying job-pair information, and on downloading job and job-pair output, to name some.