Follow-up to r33186 and r33191:  Handle the "File not found" errors (the command to execute is not in PATH) more safely.
Follow-up to r33186 and r33191:  Handle the "File not found" errors (the
command to execute is not in PATH) more safely.

* build/generator/util/executable.py
  (output):
    Don't catch all OSError exceptions.
    Check if 'cmd' is in PATH before trying to run it.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion