Default RangeEditor produces double error popups which cannot be closed

To replicate the issue run the following example and set the age value to -1.

``` python
from traits.api import HasTraits, Str, Range
from traitsui.api import View

class Person(HasTraits):
""" A simple class representing a person object.

"""
last_name = Str()

first_name = Str()

age = Range(low=0)

traits_view = View('last_name', 'first_name', 'age', resizable=True)

if __name__ == '__main__':
john = Person(first_name='John', last_name='Doe', age=42)
john.configure_traits()
```