Failed assignment to HasStrictTraits instance modifies the class.

I encountered the following behaviour while (foolishly) trying to monkeypatch a `HasStrictTraits` subclass. The fact that the monkeypatching doesn't work isn't really surprising. What _is_ suprising is that the attempt to monkeypatch one instance of the class breaks the class itself.

```
>>> from traits.api import *
>>> class A(HasStrictTraits):
... def start(self): print "Starting"
...
>>> try:
... A().start = 3.2 # Monkey-patching fails; too bad.
... except Exception:
... pass
...
>>> A().start # Surprise: the failure above affects new A instances!
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'A' object has no attribute 'start'
```

What appears to be happening is that the assignment to `A().start` creates a new class trait `start` for `A` and puts it into the `A.__class_traits__` dictionary, _before_ the failure occurs. After the failure, that new class trait still exists.