Fixes #37.

The extension replaces the error handler to catch Smarty errors while validating the settings form (i.e. the actual greeting formulas). When the code replacing the error handler was called twice, the original error handler will be lost and the extension can not restore it. In the case of an error (which seems to be a deprecation warning being handled by the error handler), this leads to an infinite loop into our error handler.

This PR guards the error handler replacement by checking whether the replacement has already been done. Also, this clears the static class member which is being used for keeping track of the original error handler when the extension restores it.