API: Use different exception type when MultiIndex indexing fails due to index not being lexsorted
I'd like to be able to separately trap errors in indexing that are due to the lack of the `MultiIndex` being lexsorted.  Right now, a `KeyError` is raised, and the only way to tell if the error was due to the lack of a lexsort versus a missing key is to look at the text of the message.

So I'd like to avoid code like this:

``` python
try:
    subdf = df.loc['A':'D','Vals']
except KeyError as ker:
    if ker.args[0].startswith("MultiIndex Slicing requires the index to be fully lexsorted"):
        print ("Need to handle fact index not sorted")
    else:
        print ("Need to handle fact that key was missing")
```

I'd rather write something like:

``` python
try:
    subdf = df.loc['A':'D','Vals']
except UnsortedIndexError:
    print ("Need to handle fact index not sorted")
except KeyError:
    print ("Need to handle fact that key was missing")
```

So could the designers accept a change where a different error is raised in case the index is not lexsorted?  If so, I'll implement it.
