GH-3247: Fix SftpSession.exists for error code (#3248)

* GH-3247: Fix `SftpSession.exists` for error code

Fixes https://github.com/spring-projects/spring-integration/issues/3247

When there is no path on the SFTP server, a `ChannelSftp.SSH_FX_NO_SUCH_FILE`
error is returned in the thrown `SftpException`.

* Fix `SftpSession.exists()` to check for the `SSH_FX_NO_SUCH_FILE` to
return `false` and re-throw an exception otherwise
* Add mock test for `SftpSession.exists()`
* Add `org.mockito.AdditionalMatchers` to `checkstyle.xml` exclusions

**Cherry-pick to 5.2.x & 5.1.x**

* * Add exists tests against Mina embedded server

src/checkstyle/checkstyle.xml

@@ -84,6 +84,7 @@
 				org.mockito.BDDMockito.*,
 				org.mockito.AdditionalAnswers.*,
 				org.mockito.ArgumentMatchers.*,
+				org.mockito.AdditionalMatchers.*,
 				org.springframework.integration.gemfire.config.xml.ParserTestUtil.*,
 				org.springframework.integration.test.mock.MockIntegration.*,
 				org.springframework.integration.test.util.TestUtils.*,