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, 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, before the temp-file
check, 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

@@ -256,10 +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 a directory it cannot list and a file whose size it
+During a look, pixa skips any file or directory in the cache that it cannot
-cannot read. It logs a warning naming the path and the error, and goes on with
+read. It logs a warning naming the path and the error, and goes on with the rest
-the rest of the look. A file renamed or removed after pixa listed its directory
+of the look. A file renamed or removed after pixa listed its directory gets no
-gets no warning.
+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

internal/imgcache/eviction_internal_test.go

@@ -948,11 +948,11 @@ func TestReconciliationGoesOnPastEntriesItCannotRead(t *testing.T) {
 	}
 }
 
-// TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead puts, in each
+// TestReconciliationWarnsOfTempFilesItCannotRead puts, in each storage
-// storage directory, a temp file whose info the pass cannot read. The
+// directory, a temp file whose info the pass cannot read. The pass must
-// pass must log a warning naming each one's path and the error, as it
+// log a warning naming each one's path and the error, as it does for any
-// does for any other file whose info it cannot read.
+// other file whose info it cannot read.
-func TestReconciliationWarnsOfTempFilesWhoseInfoItCannotRead(t *testing.T) {
+func TestReconciliationWarnsOfTempFilesItCannotRead(t *testing.T) {
 	t.Parallel()
 
 	cache, _ := newEvictionTestCache(t, 1<<30)