range-diff: libify the read_patches() function again

range-diff: libify the read_patches() function again

In library functions, we do want to avoid the (simple, but rather final)
`die()` calls, instead returning with a value indicating an error.

Let's do exactly that in the code introduced in b66885a30cb8
(range-diff: add section header instead of diff header, 2019-07-11) that
wants to error out if a diff header could not be parsed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>