Handle errors on requirement typo

[gabrielfalcao]

I quickly typed and hit enter for:

curd install markment==0/2/17

This should show a nice error rather than:

Traceback (most recent call last):
  File "/Users/gabrielfalcao/.virtualenvs/apron/bin/curd", line 9, in <module>
    load_entry_point('curdling==0.4.0', 'console_scripts', 'curd')()
  File "/Users/gabrielfalcao/.virtualenvs/apron/lib/python2.7/site-packages/curdling/tool/__init__.py", line 238, in main
    }[args.command](args)
  File "/Users/gabrielfalcao/.virtualenvs/apron/lib/python2.7/site-packages/curdling/tool/__init__.py", line 157, in get_install_command
    initial_requirements = get_packages_from_args(args)
  File "/Users/gabrielfalcao/.virtualenvs/apron/lib/python2.7/site-packages/curdling/tool/__init__.py", line 86, in get_packages_from_args
    packages = [safe_name(req) for req in (args.packages or [])]
  File "/Users/gabrielfalcao/.virtualenvs/apron/lib/python2.7/site-packages/curdling/util.py", line 49, in safe_name
    return safe_requirement(safe)
  File "/Users/gabrielfalcao/.virtualenvs/apron/lib/python2.7/site-packages/curdling/util.py", line 53, in safe_requirement
    return (util.parse_requirement(requirement).requirement
AttributeError: 'NoneType' object has no attribute 'requirement'