Add warning if benchexec -t is used with an unknown tasks-set name.
Add warning if benchexec -t is used with an unknown tasks-set name.

ultimate-pa/benchexec,dbeyer/benchexec,IljaZakharov/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,IljaZakharov/benchexec,ultimate-pa/benchexec,ultimate-pa/benchexec,martin-neuhaeusser/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,martin-neuhaeusser/benchexec,sosy-lab/benchexec,dbeyer/benchexec,martin-neuhaeusser/benchexec,martin-neuhaeusser/benchexec,IljaZakharov/benchexec,IljaZakharov/benchexec