xl: Add stat calls to keep track of ignored errors. (#4117)

Such that in a situation where all errors were
ignored we need to reduce the errors using
readQuorum to get a consistent error value.

Without this change errors generated will
never be consistent with for an expected scenario.

For example in a 6 disk setup 1 disk is missing
and 5 do not have the volume (testbucket)

Without this change Stat() would result in different
errors depending on which disk died. Can cause
confusion to S3 client application.

This change addresses need to track type of
errors we ignored and bring readQuorum to
choose the maximally occuring as the value
of truth.

cmd/object-api-listobjects_test.go

@@ -435,7 +435,7 @@ func testListObjects(obj ObjectLayer, instanceType string, t TestErrHandler) {
 		{"test-bucket-list-object", "", "", "*", 0, ListObjectsInfo{}, fmt.Errorf("delimiter '%s' is not supported", "*"), false},
 		{"test-bucket-list-object", "", "", "-", 0, ListObjectsInfo{}, fmt.Errorf("delimiter '%s' is not supported", "-"), false},
 		// Testing for failure cases with both perfix and marker (11).
-		// The prefix and marker combination to be valid it should satisy strings.HasPrefix(marker, prefix).
+		// The prefix and marker combination to be valid it should satisfy strings.HasPrefix(marker, prefix).
 		{"test-bucket-list-object", "asia", "europe-object", "", 0, ListObjectsInfo{}, fmt.Errorf("Invalid combination of marker '%s' and prefix '%s'", "europe-object", "asia"), false},
 		// Setting a non-existing directory to be prefix (12-13).
 		{"empty-bucket", "europe/france/", "", "", 1, ListObjectsInfo{}, nil, true},