Martin von Zweigbergk <martinvonz@google.com> [Thu, 22 Oct 2020 14:14:59 -0700] rev 45877
errors: introduce CanceledError and use it in a few places
This very similar to earlier patches (e.g. for `InputError`) and part
of https://www.mercurial-scm.org/wiki/ErrorCategoriesPlan.
Differential Revision: https://phab.mercurial-scm.org/D9339