ERR: better error message for trying to concat non-pandas objects
```
In [3]: pd.concat([Series(), 'foo'])
TypeError: cannot concatenate a non-NDFrame object
```

trivially ``non-NDFrame`` -> ``non-pandas``. but I think even better would be something like

```
In [4]: objs = [Series(), 'foo']

In [7]: "cannot concatenate [{pandas}] with non-pandas objects of type [{non_pandas}]".format(
   ...: pandas=','.join(set([str(o.__class__.__name__) for o in objs if isinstance(o, pandas.core.generic.NDFrame)])),
   ...: non_pandas=','.join(set([str(type(o)) for o in objs if not isinstance(o, pandas.core.generic.NDFrame)])))
Out[7]: "cannot concatenate [Series] with non-pandas objects of type [<class 'str'>]"
```