[MSCMPUB-48] unclear message when configured content == checkoutDirectory

**[Hervé Boutemy](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=hboutemy)** opened **[MSCMPUB-48](https://issues.apache.org/jira/browse/MSCMPUB-48?redirect=false)** and commented

user feedback: https://lists.apache.org/thread/s66lwdw7vhywvcdllcov4n5zbjj6s9dn

error message:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-scm-publish-plugin:3.1.0:publish-scm (default-cli) on project apache-any23: Could not copy content to SCM checkout: Source '/Users/lmcgibbn/Downloads/any23/any23-site/.svn/pristine/61/617e72262042de8cf1f11bc8e3aaa231669f181a.svn-base' and destination '/Users/lmcgibbn/Downloads/any23/any23-site/.svn/pristine/61/617e72262042de8cf1f11bc8e3aaa231669f181a.svn-base' are the same
```

is hard to understand

instead of failing when copying the first file (which is a svn file '.svn/pristine/61/617e72262042de8cf1f11bc8e3aaa231669f181a.svn-base' then creates a confusion), it would be a lot easier to understand if there was a check in the plugin that failed with an better explaination like:

```
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-scm-publish-plugin:3.1.0:publish-scm (default-cli) on project apache-any23: Configured content and checkout directory are the same: '/Users/lmcgibbn/Downloads/any23/any23-site' but should be different, see parameters documentation
```

---

**Affects:** 3.1.0

**Issue Links:**
- [ANY23-557](https://issues.apache.org/jira/browse/ANY23-557) update site publish instructions

Start at the publish-scm goal's handling of the configured content and checkoutDirectory values. Reproduce the case where both directories are the same, then make the failure identify that configuration conflict before file copying and verify that the resulting message explains the required difference.

Written by the indexing model from the issue text.