Error checking user input

[tricklynch]

User input currently is not error checked well.

• 'None' input is not checked for currently
• Out of bounds input needs to be checked for
• Duplicate player names ✔️

[tricklynch]

#5 was created to address the third bullet point

[alecbenson]

Got some code in place to deal with duplicate player names.
See d36810a