Fix AttributeError problem

[tomschr]

$ docmanager get sdafasdf
12:13:46 [DEBUG]: args: {'_ArgParser__parser': ArgumentParser(prog='docmanager', usage=None, description='Docmanager sets, gets, or analyzes meta-information for DocBook5 XML files.', formatter_class=<class 'argparse.HelpFormatter'>, conflict_handler='error', add_help=True), 'verbose': None, 'properties': None, '_ArgParser__args': None, 'action': 'get', 'files': ['sdafasdf'], 'list_all_keys': None}
Traceback (most recent call last):
  File "/local/doc/docmanager/env/bin/docmanager", line 9, in <module>
    load_entry_point('docmanager==3.0.0-beta1', 'console_scripts', 'docmanager')()
  File "/local/doc/docmanager/env/lib64/python3.4/site-packages/docmanager-3.0.0_beta1-py3.4.egg/docmanager/__init__.py", line 139, in main
    action.Actions(parser.files, parser.action, parser.arguments)
  File "/local/doc/docmanager/env/lib64/python3.4/site-packages/docmanager-3.0.0_beta1-py3.4.egg/docmanager/action.py", line 40, in __init__
    self.__files = filehandler.Files(files)
  File "/local/doc/docmanager/env/lib64/python3.4/site-packages/docmanager-3.0.0_beta1-py3.4.egg/docmanager/filehandler.py", line 44, in __init__
    sys.error("File \"" + file + "\" could not be found.")
AttributeError: 'module' object has no attribute 'error'