Include the yaml content that failed to parse

Author: timtebeek

rewrite-yaml/src/main/java/org/openrewrite/yaml/MergeYaml.java

@@ -278,6 +278,6 @@ private static Optional<Yaml.Block> maybeParse(@Language("yml") String yaml) {
 
     static Yaml parse(@Language("yml") String yaml) {
         return maybeParse(yaml)
-                .orElseThrow(() -> new IllegalArgumentException("Could not parse as YAML"));
+                .orElseThrow(() -> new IllegalArgumentException("Could not parse as YAML:\n" + yaml));
     }
 }