Add MementoOutsideWindow Exception

[Mr0grog]

Running into and fixing #53 reminded me that we don’t have a specific error for the situation where a memento can be served, but from a time that is beyond the specified target_window parameter. Instead, we currently just raise a fairly generic MementoPlaybackError.

We also have a TODO in the code that references this:

wayback/wayback/exceptions.py

Lines 32 to 40 in c066e04

	# TODO: split this up into a family of more specific errors? When playback
	# failed partway into a redirect chain, when a redirect goes outside
	# redirect_target_window, when a memento was circular?
	class MementoPlaybackError(WaybackException):
	"""
	Raised when a Memento can't be 'played back' (loaded) by the Wayback
	Machine for some reason. This is a server-side issue, not a problem in
	parsing data from Wayback.
	"""

I’m thinking MementoOutsideWindow, MementoOutOfWindow, or MementoOutOfRange are good candidates for names here. “Range” feels like more traditional wording, but doesn’t clearly match up with the parameter that causes it, which is “target_window.” We could also change the parameter name to “target_range.” ¯\_(ツ)_/¯

Regardless of the name, it should be a subclass of MementoPlaybackError.

[Mr0grog]

It would also be great if we could include info about the actual Memento’s timestamp and the specified target window, so calling code can make decisions based on or display info about how far off a viable memento might be.