registry unavailable error for 5XX status code.

Of course after exhausing all retries.

avroregistry/registry.go

@@ -198,19 +198,23 @@ func (r *Registry) doRequest(req *http.Request, result interface{}) error {
 		if err == nil {
 			return nil
 		}
-		if !attempt.More() {
-			return &UnavailableError{err}
-		}
-		if err, ok := err.(*apiError); ok && err.StatusCode != http.StatusInternalServerError {
-			// It's not a 5xx error. We want to retry on 5xx
+		if err, ok := err.(*apiError); ok {
+			// We want to retry on 5xx
 			// errors, because the Confluent Avro registry
 			// can occasionally return them as a matter of
 			// course (and there could also be an
 			// unavailable service that we're reaching
 			// through a proxy).
-			if !attempt.More() {
-				return &UnavailableError{err}
+			switch err.StatusCode {
+			case http.StatusInternalServerError:
+				if !attempt.More() {
+					return &UnavailableError{err}
+				}
+			default:
+				return err
 			}
+		}
+		if !attempt.More() {
 			return err
 		}
 	}

avroregistry/registry_test.go

@@ -340,7 +340,7 @@ func TestRetryOn500(t *testing.T) {
 	// an error.
 	failCount = 5
 	err = registry.SetCompatibility(context.Background(), "x", avro.BackwardTransitive)
-	c.Assert(err, qt.ErrorMatches, `Avro registry error \(code 50001; HTTP status 500\): Failed to update compatibility level`)
+	c.Assert(err, qt.ErrorMatches, `schema registry unavailability caused by: Avro registry error \(code 50001; HTTP status 500\): Failed to update compatibility level`)
 }
 
 func TestNoRetryOnNon5XXStatus(t *testing.T) {
@@ -392,7 +392,7 @@ func TestUnavailableError(t *testing.T) {
 	})
 	c.Assert(err, qt.Equals, nil)
 	err = registry.SetCompatibility(context.Background(), "x", avro.BackwardTransitive)
-	c.Assert(err, qt.ErrorMatches, `cannot unmarshal JSON error response from .*/config/x: unexpected content type text/html; want application/json; content: 502 Proxy Error; Proxy Error; The whole world is bogus`)
+	c.Assert(err, qt.ErrorMatches, `schema registry unavailability caused by: cannot unmarshal JSON error response from .*/config/x: unexpected content type text/html; want application/json; content: 502 Proxy Error; Proxy Error; The whole world is bogus`)
 }
 
 var schemaEquivalenceTests = []struct {