- Add error handling for waitpid returns -1 for "simple run of command"


mysql-test/lib/mtr_process.pl:
  - Add error handling for waitpid returns -1
  when a simple command is run.
  - Add missing return
  - Add mtr_errors where the program should never come 
  - Remove an else to improve program readability
  - Change mtr_debug to mtr_warning for "Got EAGAIN from fork()..."