No vertices, should give error for impossible starting vertex.
One vertex.
0

All vertices of a graph, IGRAPH_OUT:
0 1 2 3 4

1 3 4

0 1 2 3 4

3 4

4

5

All vertices of a graph, IGRAPH_IN:
0 2

0 1 2

0 2

0 1 2 3

0 1 2 3 4

5

All vertices of a graph, IGRAPH_ALL:
0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

5

All vertices of a graph, undirected:
0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

0 1 2 3 4

5

Check for invalid mode error handling.
