Observe mini-language: use "?" for optional traits

Currently the observe machinery raises an exception when there's no trait matching the given name. For example, in

```python
from traits.api import HasTraits, Instance, observe

class Foo(HasTraits):
pass

class A(HasTraits):
foo = Instance(Foo)

@observe("foo:updated")
def _do_something(self, event):
pass

a = A()
a.foo = Foo()
```

the last line gives the error:
```
ValueError: Trait named 'updated' not found on <__main__.Foo object at 0x108b7cf90>.
```

This is valuable: it catches naming errors in observe mini-language strings. However, it's not always desirable.

Proposal: allow using `observe("foo:updated?")` to match `updated` if the trait exists, but not complain if it doesn't exist. This could be especially useful in combination with `*` in something like `observe("*:updated?")`, if we re-allow use of `*` in a non-terminal position.