LRU callbacks may raise

[crusaderky]

If an LRU on_evict callback raises an exception, e.g. out of disk space, keep the key in memory instead of accidentally losing it.

Completely untested.
@ncclementi could you pick this up from here?

[ncclementi]

Thanks @crusaderky , I'll pick it up from here.

[crusaderky]

There are 3 use cases:

1. weight <= self.n. In this case, the while loop is guaranteed to stop before key is evicted.
2. weight > self.n and the sum of all other keys in self.d is enough to fill slow. so you will raise on line 72 and again on again on line 65 at some point in the middle of the while iteration. This is desirable.
3. weight > self.n and the sum of all other keys fits in slow. The while won't stop until it tries and - we know in advance - fail to store key too into slow. This can be avoided with the following:

Suggested change

	while self.total_weight > self.n:
	self.evict()
	while self.total_weight > self.n and len(self.d) > 1:
	self.evict()

[crusaderky]

Superseded by #48