main: fail on peers/peers-file flags

It is specifically not handled the same as 42, because this error is used by the flag parser:
```
: trevize etcd peers ; bin/etcd --peers-file asldkfjasdkl
invalid value "asldkfjasdkl" for flag -peers-file: flag "-peers-file" is no longer supported
```

```
: trevize etcd peers ; bin/etcd --force
2014/10/06 14:14:44 WARNING: flag "-force" is no longer supported.
2014/10/06 14:14:44 main: no data-dir provided, using default data-dir ./701739929774025524_etcd_data
```