If util.find_executable finds only files that are not executable, add them to the error message.
If util.find_executable finds only files that are not executable, add them to the error message.

This fixes #342.
The improved error message will not only be shown for test_tool_info,
but also when executing benchexec.

ultimate-pa/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,sosy-lab/benchexec,ultimate-pa/benchexec,dbeyer/benchexec,ultimate-pa/benchexec,dbeyer/benchexec