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

The walk of each storage directory returned the error of a directory it
could not list, or of a source or variant file whose info it could not
read. The pass then stopped before its later checks, every time it ran.
Now reconcileFiles skips a directory it cannot list and a file whose
info it cannot read, temp files included. It logs a warning naming the
path and the error, and the walk goes on. It reads each file's info
once and hands it to sweepStaleTempFile, reconcileVariantFile or
reconcileSourceFile. A file renamed or removed after its directory was
listed is skipped with no warning. A cancelled context still stops the
walk with its error. README.md says so under Storage.

Model: opus-5-5

README.md

@@ -256,6 +256,11 @@ 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 a directory it cannot list and a file whose size it
+cannot read. It logs a warning naming the path and the error, and goes on with
+the rest of the look. A file renamed or removed after pixa listed its directory
+gets no warning.
+
 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,