added check to see if contact name entered is valid, if not, program exits

[amandhamija98]

Issue that this pull request solves

Closes: #283

Proposed changes

This PR adds a check that ensures that the contact name entered exists in the user's address book. If it does not, the program will issue a message asking to enter a valid contact name and then exit

Types of changes

Put an x in the boxes that apply

• [ x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update (Documentation content changed)
• [ ] Other (please describe):

Checklist

Put an x in the boxes that apply

• [x ] My code follows the style guidelines of this project
• [x ] I have performed a self-review of my own code
• [x ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ x] My changes generate no new warnings

Screenshots

Other information

Any other information that is important to this pull request