Fix frozen future in 2.6 API

[halfninja]

result.get can throw an exception in various situations, like if a class definition has changed, and in this case the Promise or the resulting Future was never being completed, causing frozen requests for us when fetching old definitions from cache.

Only affects 2.6 as older code uses future.get which is wrapped in a try/catch.

[hayd]

We are also seeing this, it's pretty nasty. To confirm, with this fix, will getOrElseUpdate clear the bad data?

[halfninja]

It doesn't currently clear any data automatically but will at least reject the Future properly so that you could recover and do whatever.

[mkurz]

@mumoshu Can you have a look and merge?

[mumoshu]

@halfninja Good catch, and awesome work! Thanks for your contribution 👍