Incorrect error message when no FieldSetMapper is provided [BATCH-2848]
**[Mahmoud Ben Hassine](https://jira.spring.io/secure/ViewProfile.jspa?name=mbenhassine)** opened **[BATCH-2848](https://jira.spring.io/browse/BATCH-2848?redirect=false)** and commented

Using the `FlatFileItemReaderBuilder`, when I provide a `LineTokenizer` but no `FieldSetMapper`, the error message I get is `No LineTokenizer implementation was provided` while it should be `No FieldSetMapper implementation was provided` instead. Here is a failing test:

```java
@Test
public void testErrorMessageWhenNoFieldSetMapperIsProvided() {
	try {
		new FlatFileItemReaderBuilder<Foo>()
				.name("fooReader")
				.resource(getResource("1;2;3"))
				.lineTokenizer(line -> new DefaultFieldSet(line.split(";")))
				.build();
	} catch (IllegalStateException exception) {
		String exceptionMessage = exception.getMessage();
		if (exceptionMessage.equals("No LineTokenizer implementation was provided.")) {
			fail("Should not throw 'No LineTokenizer implementation was provided.'" + 
				 " exception when a LineTokenizer is provided");
		}
		assertEquals("No FieldSetMapper implementation was provided.", exceptionMessage);
	}
}
```

This test fails with `java.lang.IllegalStateException: No LineTokenizer implementation was provided.` which is obviously incorrect since I provided an implementation of `LineTokenizer`. In this particular case, it should rather fail with `No FieldSetMapper implementation was provided.` since I did not provide a `FieldSetMapper`.

---

**Affects:** 4.2.0, 4.0.3, 4.1.2
