fix: wrap owner-reopen File rebuild in DPYCacheError (dpycache-1)

_fresh_file_from_source wrapped the stream-copy branch's failures in
DPYCacheError but called discord.File(source.fp.name, ...) in the
owner-reopen branch with no try - a path-backed File whose backing file was
deleted between caching calls raised a raw FileNotFoundError/PermissionError
where the module's fail-loud contract promises DPYCacheError for every
lib-domain fault. Wrap the reopen the same way the stream-copy branch
already is.

Signed-off-by: disqualifier <dev@disqualifier.me>

src/dpy_cache/dpy_cache.py

@@ -255,12 +255,18 @@ class DPYCache:
         seeks to the original position and copies the bytes for a caller-supplied stream;
         raises DPYCacheError if the underlying stream can't be safely re-read"""
         if source._owner and hasattr(source.fp, "name"):
-            return discord.File(
-                source.fp.name,
-                filename=name,
-                spoiler=source.spoiler,
-                description=source.description,
-            )
+            try:
+                return discord.File(
+                    source.fp.name,
+                    filename=name,
+                    spoiler=source.spoiler,
+                    description=source.description,
+                )
+            except OSError as err:
+                raise DPYCacheError(
+                    f"discord.File source for {name!r} could not be reopened from "
+                    f"{source.fp.name!r} (backing file missing or unreadable)"
+                ) from err
         if source.fp.closed:
             raise DPYCacheError(
                 f"discord.File source for {name!r} is already closed and can't be safely "