vmbuilder throws exception if it doesn't find a helper program

[Andreas Heck]

If vmbuilder misses any of its dependent helper programs like grub or qemu it throws an exception of type OSError like this one:

Traceback (most recent call last):
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/vmbuilder", line 29, in <module>
    VMBuilder.run()
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/__init__.py", line 65, in run
    frontend.run()
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/plugins/cli/__init__.py", line 70, in run
    vm.create()
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/vm.py", line 483, in create
    self.install()
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/vm.py", line 416, in install
    self.distro.install_bootloader()
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/plugins/ubuntu/distro.py", line 173, in install_bootloader
    EOT''')
  File "/home/ahe/Desktop/Projekte/VMbuilder/vmbuilder-gui/VMBuilder/util.py", line 105, in run_cmd
    proc = subprocess.Popen(args, stdin=stdin_arg, stderr=subprocess.PIPE, stdout=subprocess.PIPE, env=proc_env)
  File "/usr/lib/python2.6/subprocess.py", line 595, in __init__
    errread, errwrite)
  File "/usr/lib/python2.6/subprocess.py", line 1092, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

This exception should be catched and vmbuilder should print an error message to stderr that tells the user which program is missing.