debug missing script engine

Author: tisoft

lib/src/main/java/com/diffplug/spotless/generic/Jsr223Step.java

@@ -17,6 +17,7 @@
 
 import java.io.Serializable;
 import java.util.Objects;
+import java.util.stream.Collectors;
 
 import javax.script.ScriptEngine;
 import javax.script.ScriptEngineManager;
@@ -61,6 +62,11 @@ FormatterFunc toFormatter() {
 			}
 			ScriptEngine scriptEngine = scriptEngineManager.getEngineByName(engine);
 
+			if (scriptEngine == null) {
+				throw new IllegalArgumentException("Unknown script engine. Available engines: " +
+						scriptEngineManager.getEngineFactories().stream().flatMap(f -> f.getNames().stream()).collect(Collectors.joining(", ")));
+			}
+
 			// evaluate JavaScript code
 			return raw -> {
 				scriptEngine.put("source", raw);