wait_until: Add ignored_exceptions parameter

[drothlis]

Useful if you've started an HTTP server in a background thread and
you're waiting for it to start up by trying to connect to it; or you've
just sent a key-press to the device-under-test and the FrameObject
you're waiting for doesn't handle unknown states very well; etc.

TODO:

• Merge stbt.wait_until: Add predicate parameter #444.
• Test unicode behaviour in exception logging.
• Use logging with excinfo=True.
• Add test that it raises if the callable continues to raise exceptions after the timeout.

[drothlis]

N.B. The name is consistent with Selenium's WebDriverWait which also has an ignored_exceptions parameter.

[drothlis]

Rebased on top of #444.

[drothlis]

Rebased onto master after merging #444.

[wmanley]

This works?!? Fancy that, I guess it makes sense.

[wmanley]

Could this be

try:
    value = callable_()
    predicate_value = predicate(value)
except ...

and then you could avoid the else: block and the second try...except?

Unless this has something to do with not setting value = None if predicate fails?

[wmanley]

i'd like a little bit more discussion of use-cases here. Can you provide an example of where this would have been useful?

[drothlis]

We'll need to test that it raises (or returns None?) if the callable continues to raise exceptions after the timeout.