Fail the build on missing Override annotation [LUCENE-6967]

Spin-off of #8014. Even though javac does not support it, ecj does with this one-liner:

```
--- lucene/tools/javadoc/ecj.javadocs.prefs (revision 1723604)
+++ lucene/tools/javadoc/ecj.javadocs.prefs (working copy)
`@@` -47,7 +47,7 `@@`
org.eclipse.jdt.core.compiler.problem.missingJavadocTagsMethodTypeParameters=disabled
org.eclipse.jdt.core.compiler.problem.missingJavadocTagsOverriding=disabled
org.eclipse.jdt.core.compiler.problem.missingJavadocTagsVisibility=public
-org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotation=ignore
+org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotation=error
org.eclipse.jdt.core.compiler.problem.missingOverrideAnnotationForInterfaceMethodImplementation=enabled
org.eclipse.jdt.core.compiler.problem.missingSerialVersion=ignore
org.eclipse.jdt.core.compiler.problem.missingSynchronizedOnInheritedMethod=ignore
```

---
Migrated from [LUCENE-6967](https://issues.apache.org/jira/browse/LUCENE-6967) by Adrien Grand (@jpountz), updated Jan 12 2016
Attachments: [LUCENE-6967.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-6967/LUCENE-6967.patch) (versions: 2)