Revert 276acd1

This commit reverts the change to `StepLocator` as it introduces
an inconsistency with `JobLocator` (one returns null and the other
throws an exception when no step/job is found with the given name).

spring-batch-core/src/main/java/org/springframework/batch/core/step/StepLocator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2009-2018 the original author or authors.
+ * Copyright 2009 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,20 +18,17 @@
 import java.util.Collection;
 
 import org.springframework.batch.core.Step;
-import org.springframework.lang.Nullable;
 
 /**
  * Interface for locating a {@link Step} instance by name.
  * 
  * @author Dave Syer
- * @author Mahmoud Ben Hassine
  *
  */
 public interface StepLocator {
 
 	Collection<String> getStepNames();
-
-	@Nullable
-	Step getStep(String stepName);
+
+	Step getStep(String stepName) throws NoSuchStepException;
 
 }