reftable/record: stop using `BUG()` in `reftable_record_init()`

reftable/record: stop using `BUG()` in `reftable_record_init()`

We're aborting the program via `BUG()` in case `reftable_record_init()`
was invoked with an unknown record type. This is bad because we may now
die in library code, and because it makes us depend on the Git codebase.

Refactor the code such that `reftable_record_init()` can return an error
code to the caller. Adapt any callers accordingly.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>