Added validation to arguments

Added an if/else statement inside each function to check if users are
passing numbers. If not, an error message is the output.
Added validation to divide by 0 and edited echo statements

Nested another if/else inside divide to make sure user is not dividing
by 0– outputs error message. Edited echo statements in each function to
match their appropriate math operation.
Updated error messages to show values

Changed the error messages in each function to show the values that
were passed by the user.
Refactored the error messages into a function

Wrote validate function, added it to other functions. If any numbers
aren’t passed, user is kicked out to try again.