bugfixes: arithmetic error handling

This is a hotfix for arithmetic error handling. As it stands, three arithmetic errors, `OverflowError`, `ZeroDivisionError`, and the error for negative natural log, all throw errors with standard print(), which is immediately cleared. From the most recent commit:
- OverflowError is now handled properly; values are deleted from the
stack.
- Operators can now return a value, which is assigned to an error if it
is not the empty string, None, or '\n'. This fixes arithmetic errors
(can't ln a negative; can't divide by zero).