Add warning if benchexec -r is used with unknown run definition name.
Add warning if benchexec -r is used with unknown run definition name.

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