Update to match current sklearn dist_metric.pyx per issue #222

Author: lmcinnes

hdbscan/dist_metrics.pyx

@@ -1137,7 +1137,14 @@ cdef class PyFuncDistance(DistanceMetric):
         cdef np.ndarray x2arr
         x1arr = _buffer_to_ndarray(x1, size)
         x2arr = _buffer_to_ndarray(x2, size)
-        return self.func(x1arr, x2arr, **self.kwargs)
+        d = self.func(x1arr, x2arr, **self.kwargs)
+        try:
+            # Cython generates code here that results in a TypeError
+            # if d is the wrong type.
+            return d
+        except TypeError:
+            raise TypeError("Custom distance function must accept two "
+                            "vectors and return a float.")
 
 
 cdef inline double fmax(double a, double b) nogil: