Add exception handling to ensure users can't enter non-integers

Add an exception that will stop the system from breaking when a non-integer is entered into the console.

https://github.com/2002-feb24-net/morgan-project0/blob/c13bac5a6d6e3b2856925f7d5539c685c8b9afa0/InstrumentStore.Library/Model/Customer.cs#L125

The issue points to line 125 in InstrumentStore.Library/Model/Customer.cs. Start by examining that file to see how user input is currently handled. Look for a method that reads console input and parse it as an integer. Add a try-catch block or use int.TryParse to validate the input and provide a helpful error message. Test by running the console application and entering non-integer values to ensure the system no longer breaks.

索引モデルが issue の本文から書いたものです。