Figure out how to handle failure outside of task context

[brson]

Currently the runtime aborts. We can probably do better.

[catamorphism]

Assigning to @brson -- de-milestone/de-assign if necessary.

[brson]

Consider this related to #4419. I have some progress on this and generally know what I want to do. In the new scheduler I have a type called LocalServices that contains all the runtime capabilities required by the language itself. Tasks are one provider of LocalServices but there can be other.

This isn't a priority for me though. Demilestoning.

[metajack]

revisitng for triage; nothing to add

[metajack]

triage bump

[flaper87]

triage bump

[alexcrichton]

This was changed recently in 0c7c93b to simply attempt to unwind. If there is no "catch frame" available, only then do we abort. I believe that this is about as good as we can get, so I'm going to close this.