Clarifies error message in eccentricity
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.
Sixshaman/networkx,jfinkels/networkx,NvanAdrichem/networkx,andnovar/networkx,OrkoHunter/networkx,JamesClough/networkx,goulu/networkx,SanketDG/networkx,michaelpacer/networkx,cmtm/networkx