scripts/silenant.py: Fix error message if ant not found

If Apache Ant is not found, the build fails with a cryptic error
message:

  [penberg@localhost osv]$ make
    ANT tests/bench
  Traceback (most recent call last):
    File "scripts/silentant.py", line 14, in <module>
      stderr = subprocess.PIPE)
    File "/usr/lib64/python2.7/subprocess.py", line 711, in __init__
      errread, errwrite)
    File "/usr/lib64/python2.7/subprocess.py", line 1308, in _execute_child
      raise child_exception
  OSError: [Errno 2] No such file or directory
  make: *** [all] Error 1

Make it more human friendly:

  [penberg@localhost osv]$ make
    ANT tests/bench
  Apache Ant not found. Please install the 'ant' package.
  make: *** [all] Error 1

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>