Report handler panics through the logger and answer 500 (closes #187)

internal/middleware/accesslog_test.go

@@ -376,7 +376,7 @@ func lineSizeCases() map[string]sizeCase {
 
 	// The url field on a 5xx keeps the concrete path, so it reaches its
 	// own budget on the same line as the three header fields. That is
-	// the widest line the service can be made to write.
+	// the widest access log line the service can be made to write.
 	longPath := "/boom/" + strings.Repeat("x", oversizedSegmentBytes)
 	wantLongURL := longPath[:maxFieldBytes] + truncationSuffix