5.4 - set_error_handler to restore_error_handler

set_error_handler(null) is >= 5.5

restore_error_handler() is 5.4 compatible and pops the most recent error handler off the stack, which appears to serve the same intended effect
Cleaned up unused variable
Merge pull request #34 from jamesshannon/patch-1

5.4 - set_error_handler to restore_error_handler