Change error level to warning.

Co-authored-by: Weston Ruter <westonruter@git.wordpress.org>

Author: dmsnell

src/wp-includes/html-api/class-wp-html-processor.php

@@ -1115,7 +1115,7 @@ public function serialize(): ?string {
 			wp_trigger_error(
 				__METHOD__,
 				"An HTML Processor which has already started processing cannot serialize it's contents. Serialize immediately after creating the instance.",
-				E_USER_ERROR
+				E_USER_WARNING
 			);
 			return null;
 		}
@@ -1223,7 +1223,7 @@ public function serialize(): ?string {
 			wp_trigger_error(
 				__METHOD__,
 				"Cannot serialize HTML Processor with parsing error: {$this->get_last_error()}.",
-				E_USER_ERROR
+				E_USER_WARNING
 			);
 			return null;
 		}