bugfixes: arithmetic error handling

- 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).
Merge pull request #4 from qguv/hotfix

hotfixes: arithmetic error handling