rhg: Propogate manifest parse errors instead of panicking...
rhg: Propogate manifest parse errors instead of panicking
The Rust parser for the manifest file format is an iterator. Now every item
from that iterator is a `Result`, which makes error handling required
in multiple new places.
This makes better recovery on errors possible, compare to a run time panic.
Differential Revision:
https://phab.mercurial-scm.org/D11771
