BundledExchangesParser: Introduce structs for parse errors

This is a preparation for supporting the new bundle format [1].

After this change, ParseMetadata() and ParseResponse() return error as
Bundle{Metadata,Response}ParseError struct. At the moment these structs
are just wrappers of string, but additional fields will be added in
follow up CLs.

[1] https://github.com/WICG/webpackage/pull/450

Bug: 969596
Change-Id: I5bdbc84149f818f07470999fc42c8680e46f2385
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1724150
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683100}
13 files changed