zodbcommit: Provide full context when reporting errors

In the previous patch we taught object copy handler to report more
details, but it was still incomplete - the error was missing details
about which operation was run - commit, or restore of particular
transaction.

Noting that it can be also noted that other errors reported from that
function lack such context.

-> So fix it universally, at least for zodbcommit for now: set top-level
runctx to topic of what we are doing, and use that runctx when
generating errors. Runctx describes what we are running, and could be
also later used for logging and tracing. That's why it is called runctx
instead of just errctx for "error context".

TODO currently it is only exceptions that we explicitly raise which get
the context. If an exception is raised by something that we call - the
context won't be added. It would be good to later rework error handling
and append such context for any raised error. Defer and
https://lab.nexedi.com/kirr/go123/blob/863c4602/xerr/__init__.py has
something preliminary for this.

The particular error when restoring a missing object copy becomes

    ValueError: /tmp/demo002868462/δ0285cbac75555580/δ.fs: restore 0285cbacb70a3db3 @0285cbacb258bf66: object 0000000000000003: copy from @0285cbac70a3d733: no data

instead of older

    ValueError: /tmp/demo358030847/δ0285cbac75555580/δ.fs: object 0000000000000003: copy from @0285cbac70a3d733: no data

/reviewed-by @jerome
/reviewed-on nexedi/zodbtools!20