Moved PackageNotFoundException into its own file, began preparations for Type Checking Error Handling by passing most type checking errors to the ErrorHandler.
Improved type checking error messages across a variety of cases.
Improved error location precision in some type checking cases.
Improved error messages for cases where an operator couldn't be used with the type of its lhs or rhs expression.
Fixed minor errors.
Refactored error handling code to remove redundant method/enum.
Merge pull request #4 from DanielJCampbell/master

Fixed up type checking errors