Answer 502 for an upstream host with no address or a failed fetch (closes #367)

The image routes answered 500 and logged at error level when the upstream
host had no address, or when the fetch failed before the upstream
answered, such as on a refused connection. The fetcher now marks such a
failure with ErrUpstreamRequestFailed; a cancelled fetch is not marked.
respondImageError answers it, and ErrInvalidHost, 502 "upstream error"
and logs them at warn level. README Routes lists both under 502.

Model: opus-5-5

internal/handlers/image_errors_internal_test.go

@@ -12,6 +12,7 @@ import (
 	"net/http/httptest"
 	"slices"
 	"strings"
+	"syscall"
 	"testing"
 	"testing/fstest"
 	"time"
@@ -461,6 +462,13 @@ func TestImageRoutes_SameErrorAnswers(t *testing.T) {
 		{"upstream error status",
 			fmt.Errorf("%w: status 404", httpfetcher.ErrUpstreamError),
 			http.StatusBadGateway, slog.LevelWarn},
+		{"upstream host with no address",
+			fmt.Errorf("%w: cdn.example.com", httpfetcher.ErrInvalidHost),
+			http.StatusBadGateway, slog.LevelWarn},
+		{"connection refused",
+			fmt.Errorf("%w: %w", httpfetcher.ErrUpstreamRequestFailed,
+				syscall.ECONNREFUSED),
+			http.StatusBadGateway, slog.LevelWarn},
 		{"no response headers within the fetch timeout",
 			httpfetcher.ErrUpstreamTimeout, http.StatusGatewayTimeout,
 			slog.LevelWarn},