Option to use fallback learner only for timeout, not for errors

[mb706]

Under some circumstances, we may want to use the fallback learner when a timeout happens, but would still want things to abort when another ("actual") error happens.

[sebffischer]

When we call learner$encapsulate(...) we want to be able to specify the error classes for we trigger the fallback.
By default, all errors should be captured.

[sebffischer]

We need to add proper classes to the conditions for this.

[sebffischer]

a3356c1