Checkbox doesn't eval any requirements when a registry fails

When Checkbox attempts to eval requirements, it uses the registry to interpret variable names. However, if a command registry fails, the following traceback might be generated:

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/checkbox/reactor.py", line 89, in fire
    results.append(handler(*args, **kwargs))
  File "/usr/share/checkbox/plugins/registry_info.py", line 31, in gather
    registry_eval_recursive(self._manager.registry, "False")
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 180, in registry_eval_recursive
    values.extend(registry_eval_recursive(value, source, mask))
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 180, in registry_eval_recursive
    values.extend(registry_eval_recursive(value, source, mask))
  File "/usr/lib/python2.6/dist-packages/checkbox/registry.py", line 178, in registry_eval_recursive
    for key, value in registry.items():
  File "/usr/share/checkbox/registries/gconf.py", line 43, in items
    lines[-1] += line
IndexError: list index out of range

The problem is that the status of running a command registry is not checked. So, in the event the gconftool-2 command is not found in the above gconf registry example, then the string "Command not fond" is returned and then parsed. Instead, the status of the command should be checked.