Refuse an unparseable exp with 400; log swallowed cache errors (closes #72)

An exp that was not a whole number, or empty, was ignored, so a URL for
a host that needs a signature got 401 as if it had no exp. It is now a
400 naming exp and the value; only an exp missing from the URL is
unchanged.

A failed variant .meta write, source metadata JSON write, Stats count
query, stats counter update, negative cache write or expired negative
cache delete was discarded without a trace. Each is now logged at warn
with the path or key and the error, and stays non-fatal. VariantStorage
takes the cache's logger for this. The metadata JSON write moved out of
StoreSource into writeMetadataSidecar to keep StoreSource within the
function length limit.

Model: opus-5-5

README.md

@@ -127,7 +127,9 @@ Where:
 - `width` — requested width in pixels, `0` for original
 - `height` — requested height in pixels, `0` for original
 - `format` — output format (jpeg, png, webp, avif, gif, orig)
-- `expiration` — Unix timestamp when signature expires
+- `expiration` — the URL's `exp` query parameter, the Unix timestamp when
+  the signature expires; a request whose `exp` is not a whole number, an
+  empty `exp=` included, is refused with 400
 - `quality` — the URL's `q` query parameter, a whole number from 1 to 100,
   or `85` when the URL has no `q`; a request whose `q` is anything else is
   refused with 400