Update: graph_matrix

[priyaa-rawat]

Describe your change:

Did some changes by implementing if conditions to prevent "IndexOverflow" error and added a function to remove an edge.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• [x ] I know that pull requests will not be merged if they fail the automated tests.
• [x ] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming #conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.
  Hacktoberfest 2021 - Implement algorithms or data structures, fix existing ones and more! #4996 @cclauss @poyea

[priyaa-rawat]

@cclauss is printing error messages through exceptions allowed?

        try:
            self.graph[u - 1][v - 1] = 1
            self.graph[v - 1][u - 1] = 1
        except IndexError as e:
            print(e)
            raise

[cclauss]

As long as your raise after you print() like I added above.

Or better yet, raise IndexError("This is my even better error message.")