Make ps -o error reporting intelligible.

I actually thought ps was segfaulting, and it wasn't until I was in gdb
that I even noticed there was an error message at all, and even then I
had to read the source to work out what it was trying to tell me.

Before:
  $ ps -o user,pid,ppid,vsize,rss,nice,wchan,addr,unknown,stat,cmd
  ps: -o 'user,pid,ppid,vsize,rss,nice,wchan,addr,unknown,stat,cmd'@41
  USER       PID  PPID    VSZ   RSS  NI WCHAN          ADDR$

After:
  $ ps -o user,pid,ppid,vsize,rss,nice,wchan,addr,unknown,stat,cmd
  ps: bad -o field 'user,pid,ppid,vsize,rss,nice,wchan,addr,unknown,stat,cmd'
                                                            ^

0 files changed, 0 insertions, 0 deletions