fix conflicting plugins error message

Print out a more correct message when a plugin is marked as both enabled and disabled.
Previously:
```
File "/home/vagrant/dxr/dxr/config.py", line 101, in __init__
    print >> sys.stderr, msg % (p, name)
UnboundLocalError: local variable 'p' referenced before assignment
```
Now:
```
Plugin: 'pygmentize' is both enabled and disabled
Plugin: 'clang' is both enabled and disabled
```