Go on past cache entries the reconciliation pass cannot read (closes #343)

The walk of each storage directory returned the error of an entry it
could not read. The pass then stopped before its later checks, and the
same entry stopped every later pass. Now reconcileFiles skips a
directory it cannot list and a file whose info it cannot read. It logs
a warning naming the path and the error, and the walk goes on. It reads
each file's info itself and hands it to reconcileVariantFile and
reconcileSourceFile. A cancelled context still stops the walk with its
error. README.md says so under Storage.

Model: opus-5-5

README.md

@@ -256,6 +256,10 @@ Each look also drops the rows of variants whose files are missing. When pixa
 cannot read a variant file's size for another reason, it keeps the variant's
 row. It logs a warning naming the variant key and the error.
 
+During a look, pixa skips any file or directory in the cache that it cannot
+read. It logs a warning naming the path and the error, and goes on with the rest
+of the look.
+
 pixa's target is 1-5k r/s, which has not been measured at that rate (see Load
 Test). Toward it, a cache hit opens the variant's file and nothing else, and
 makes no database call. The content type comes from the file's first bytes,