[MNG-7351] Improve javadoc for MojoExecutionException and MojoFailureException

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNG-7351](https://issues.apache.org/jira/browse/MNG-7351?redirect=false)** and commented

From the javadoc the semantical difference is not clear, but https://books.sonatype.com/mvnref-book/reference/writing-plugins-sect-custom-plugin.html#writing-plugins-sect-failure gives further hints, which should be also reflected in the Javadoc of both https://github.com/apache/maven/blob/master/maven-plugin-api/src/main/java/org/apache/maven/plugin/MojoExecutionException.java and https://github.com/apache/maven/blob/master/maven-plugin-api/src/main/java/org/apache/maven/plugin/MojoFailureException.java. Compare also with https://stackoverflow.com/questions/1033154/when-to-use-mojoexecutionexception-vs-mojofailureexception-in-maven

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**Affects:** 3.8.4

**Issue Links:**
- [SUREFIRE-1835](https://issues.apache.org/jira/browse/SUREFIRE-1835) failsafe plugin's verify goal does not honor `mvn --fail-at-end`

**Remote Links:**
- [GitHub Pull Request #632
](https://github.com/apache/maven/pull/632)