Revert "[engine] Don't catch cancellations in except"

This reverts commit b2ab5f39fa6f3b4d44e069e3da9009e3050f89eb.

Reason for revert: breaks build roller

Original change's description:
> [engine] Don't catch cancellations in except
>
> Steps might be cancelled because they're in a self.m.time.timeout()
> context, or because the overall build is being cancelled. The first case
> is much like a failed step, and the caller likely wants to handle it as
> a failed step. The latter case is not like a failed step, but more like
> a KeyboardInterrupt. KeyboardInterrupt doesn't inherit from Exception,
> so it's not caught by 'except Exception:' blocks. It inherits from
> BaseException. This ensures that when a user hits Ctrl-C, very few
> exception handlers are invoked to process the KeyboardInterrupt, and the
> process simply exits.
>
> Add a new exception type, CancelledBuild, that inherits directly from
> BaseException. Add handling of this exception type in a couple places,
> but mostly let it propagate untouched. This ensures that a cancelled
> build won't be processed by an exception handler that doesn't understand
> that the build has been cancelled.
>
> Example failure: http://ci.chromium.org/b/8699098381363633537/infra
>
> Bug: b/456536120
> Change-Id: Iab371e7ada59b035efbddfe781a9e44017f7f463
> Recipe-Manual-Change: build
> Recipe-Manual-Change: chromiumos
> Recipe-Manual-Change: fuchsia
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/7120818
> Commit-Queue: Rob Mohr <mohrr@google.com>
> Reviewed-by: Robbie Iannucci <iannucci@google.com>

Bug: b/456536120
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Idd6a5ba72e625b9c99ad56686f2402a1bdd2ce4c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/7124691
Reviewed-by: Robbie Iannucci <iannucci@google.com>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Robbie Iannucci <iannucci@google.com>
Auto-Submit: Rob Mohr <mohrr@google.com>