Epic: Improve Graph Read/Write Error Detection and Reporting

[john-boyer-phd]

There are several improvements that can be made throughout the graphIO.c code:

• All instances of return NOTOK and RetVal = NOTOK should be preceded with an explanatory ErrorMessage() to indicate what has gone wrong, in keeping with the level of information now provide by the new "graph6"reader. For example, there should be an error message in _ReadAdjList() that indicates loop edges are not supported.
• The adjacency matrix reader should be amended to detect the failure of fscanf() and sscanf() as well as successful reading of integer values other than 0 and 1, and provide an appropriate ErrorMessage() and return NOTOK when such events occur.
• Amend ReadAdjList() and _ReadLEDAGraph() to efficiently detect and report an error and return NOTOK if multiple edges are detected
• Amend the LEDA graph format reader to detect a loop edge and report an error and return NOTOK
• Amend _WriteAdjMatrix() and the new "graph6" writer to report an error and return NOTOK if the graph being written is a digraph.