Improve snakeyaml error message (#400)

* Improve snakeyaml error message

This provides a better error message for the requirements to parse yaml files
that clearly indicate what to change in order to support it.

* Format

serving/src/main/java/ai/djl/serving/workflow/WorkflowDefinition.java

@@ -119,7 +119,12 @@ public static WorkflowDefinition parse(URI uri, Reader input) {
                 String asJson = GSON.toJson(yaml);
                 return GSON.fromJson(asJson, WorkflowDefinition.class);
             } catch (ReflectiveOperationException e) {
-                throw new IllegalArgumentException("Yaml parsing is not supported.", e);
+                throw new IllegalArgumentException(
+                        "Yaml parsing is not supported. In order to support parsing Yaml files, the"
+                                + " dependency snakeyaml is required. Please add"
+                                + " 'org.yaml.snakeyaml.Yaml' to your classpath, pom.xml, or"
+                                + " build.gradle.",
+                        e);
             }
         } else if (fileName.endsWith(".json")) {
             return GSON.fromJson(input, WorkflowDefinition.class);