Added error handling to DigraphRemoveEdge(s) and DigraphRemoveVertex(ices) operations. Also added tests for (#issue863)

[devansh2605]

For issue - #863

I've added error validation to DigraphRemoveVertex(ices) and DigraphRemoveEdge(s) operations (both mutable and immutable versions). It now gives an error when trying to remove non-existent vertices or edges. I have also updated the tests which ignored or didnt take non- existent vertices or edges into consideration and have added new tests as well.

[mtorpey]

This is failing due to tests that are now failing in the documentation examples. The examples that are failing are in oper.xml starting on line 716 and 800:

########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:716
# Input is:
new := DigraphRemoveEdge(D, [25000, 2]);;
# Expected output:
# But found:
Error, the edge [25000, 2] does not exist,
########
########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:716
# Input is:
new = D;
# Expected output:
true
# But found:
false
########
########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:716
# Input is:
new := DigraphRemoveEdge(D, 2500, 2);;
# Expected output:
# But found:
Error, the edge [2500, 2] does not exist,
########
########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:716
# Input is:
IsIdenticalObj(new, D);
# Expected output:
true
# But found:
false
########
# Running example 32   . . . msecs: 2
# Running example 33   . . . msecs: 84
########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:764
# Input is:
new := DigraphRemoveEdges(D, [[1, 2], [2, 3], [3, 100]]);
# Expected output:
<mutable digraph with 250000 vertices, 249997 edges>
# But found:
Error, the edge [3, 100] does not exist,
########
########> Diff in:
# /tmp/gaproot/pkg/Digraphs/doc/oper.xml:764
# Input is:
new = D;
# Expected output:
true
# But found:
false
########