iwa: Rename `ReadError` to `ReadIntegrityBlockAndMetadataError`

This change improves clarity, because the two error types used by the
`SignedWebBundleReader` now accurately describe their use cases:
- `ReadIntegrityBlockAndMetadataError` is the type used for errors
  occurring during integrity block and metadata parsing.
- `ReadResponseError` is the type used for errors occurring during
  response parsing.

Bug: 1382396
Change-Id: Ice6814a4295fb6d398d558d72e127302bdf4f397
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4020157
Commit-Queue: Christian Flach <cmfcmf@chromium.org>
Reviewed-by: Phillis Tang <phillis@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1070916}