Add to cache read failure log 'falling back to S3' (#1245)

When a cache read fails, there have been open questions in support cases
about if that will have then failed the whole read or asked S3. This is
a simple change to address that ambiguity.

### Does this change impact existing behavior?

Simple logging change only.

### Does this change need a changelog entry? Does it require a version
change?

No, simple log content change.

---

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(DCO)](https://developercertificate.org/).

Signed-off-by: Daniel Carl Jones <[email protected]>

Author: dannycjones

mountpoint-s3/src/prefetch/caching_stream.rs

@@ -170,7 +170,7 @@ where
                     block_index,
                     ?range,
                     ?error,
-                    "error reading block from cache",
+                    "error reading block from cache, falling back to S3",
                 ),
             }
             // If a block is uncached or reading it fails, fallback to S3 for the rest of the stream.