Fix KeyError on doit list --status when missing file dependency

[poliquin]

Tasks missing a file dependency will cause a KeyError on doit list --status because List.STATUS_MAP lacks an 'error' key.

This pull requests resolves the issue by showing 'E' next to the task name.

Example of a problematic dodo.py that causes a KeyError on doit list --status before this fix:

def task_a():
    """Task A"""
    return {
        'actions': ['echo "hello" > a.txt'],
        'file_dep': ['b.txt']  # assume b.txt does not exist
    }

[coveralls]

Coverage increased (+0.0003%) to 99.79% when pulling 188dfd2 on poliquin:list_err into 234d9bd on pydoit:master.

[schettino72]

thanks. squashed and merge. sorry for delay.

You should add yourself to the AUTHORS file

[coveralls]

Coverage increased (+0.0009%) to 99.79% when pulling 188dfd2 on poliquin:list_err into 234d9bd on pydoit:master.

[schettino72]

really merged (and pushed) this time. sorry.