Well, I'll be.  I cannot find any other example of that in the scikit!  I think something like (and this is entirely made up--you'll know much better what it should look like, I believe):

```
try:
    sublib.func()
except TypeError:
    raise ValueError("Float images not supported. Please pre-label image first using skimage.morphology.label.")
```

If ``subflib.func()`` can raise multiple errors of the same type, you could (although it's not desired) also check explicitly for the one you want, and only re-raise if that is not the case:

```
try:
    sublib.func()
except TypeError as e:
    if 'float' not in e.message:
        raise
    else:
        raise ValueError("Scikit-image-specific error message.")
```