Clarify fetching targets with multiple hashes if one is missing

In https://theupdateframework.github.io/specification/v1.0.26/#fetch-target, when downloading targets with consistent snapshots enabled, it states:

... Otherwise, the filename is of the form HASH.FILENAME.EXT (e.g., c14aeb4ac9f4a8fc0d83d12482b9197452f6adf3eb710e3b1e2b79e8d14cb681.foobar.tar.gz), where HASH is one of the hashes of the targets file listed in the targets metadata file found earlier in step § 5.6 Update the targets role. In either case, the client MUST write the file to non-volatile storage as FILENAME.EXT.

Consider the case where we have a consistent snapshot repository, and a target foo with both sha-256 and sha-512 hashes listed. As written, it sounds like we we pick one of the entries, say the sha-256 and fetch $SHA256.foo, but it doesn't exist. As written, it sounds like we should give up, but $SHA512.foo might exist. Should we try to download it as well? go-tuf, for example, will try to download every hash prefixed version.

Read the §5.6 fetch-target section in specification v1.0.26 and compare its wording with go-tuf's client/client.go around line 569. Determine and document whether clients should try each hash-prefixed target when one is missing, including the expected behavior when none can be fetched.