Clarifies error message in eccentricity

Changes the error message when an infinite path length is encountered so
that it is different based on whether the graph is directed or not.

networkx/algorithms/distance_measures.py

@@ -59,9 +59,14 @@ def eccentricity(G, v=None, sp=None):
             except TypeError:
                 raise networkx.NetworkXError('Format of "sp" is invalid.')
         if L != order:
-            msg = "Graph not connected: infinite path length"
+            if G.is_directed():
+                msg = ('Found infinite path length because the digraph is not'
+                       ' strongly connected')
+            else:
+                msg = ('Found infinite path length because the graph is not'
+                       ' connected')
             raise networkx.NetworkXError(msg)
-            
+
         e[n]=max(length.values())
 
     if v in G: