Edited Error Messages and Arithmetic Expression Evaluation

Evaluated Arithmetic Expressions and Added newline to error messages
Multiplication, Division and Modulo

Corrected some bugs in Multiplication, Division and Modulo
Fixed newline for errors

Added Environment.NewLine as newline character for displaying error
messages in the textbox
Printing with newline

Added printing with newline and added "else" to if statements for code
optimization purposes
Subtraction Fixed

Fixed bug in evaluating subtraction operators