Switch Jenkins.getInstance() to @Nonnull (#2297)

As detailed by @stephenc:

> So there is `getInstanceOrNull()` which is for use in Jenkins core or
> in code that plugins add that may escape the Jenkins singleton
> lifecycle.
>
> In regular plugins `getInstance()` will never return null as the
> lifecycle does not instantiate them until after there is a Jenkins
> singleton and the plugins will be stopped before there is no
> singleton.
>
> The `getActiveInstance()` is therefore equivalent to `getInstance()`
> and should never have been born.
>
> The only way a plugin can get a `null` from `Jenkins.getInstance()` is
> if you install an `atExit()` handler, use a `PhantomReference` type
> thing, or directly manipulate the servlet container in some way or
> other. A correctly written plugin should unwind any of those things
> when it is stopped or a method bound to the termination lifecycle. If
> there are some cases (for which I cannot anticipate a good coder being
> able to write a correct termination lifecycle method to fix) where the
> code cannot be unhooked... then in those cases the plugin should use
> `getInstanceOrNull()` and know how to respond to the `null` that could
> not be avoided.

core/src/main/java/jenkins/model/Jenkins.java

@@ -754,10 +754,17 @@ public class Jenkins extends AbstractCIBase implements DirectlyModifiableTopLeve
      * @throws IllegalStateException {@link Jenkins} has not been started, or was already shut down
      */
     @CLIResolver
-    @Nullable // TODO replace with non-null once Jenkins 2.0+
+    @Nonnull
     public static Jenkins getInstance() {
-        // TODO throw an IllegalStateException in Jenkins 2.0+
-        return HOLDER.getInstance();
+        Jenkins instance = HOLDER.getInstance();
+        if (instance == null) {
+            if(!Boolean.getBoolean(Jenkins.class.getName()+".disableExceptionOnNullInstance")) {
+                // TODO: remove that second block around 2.20 (that is: ~20 versions to battle test it)
+                // See https://github.com/jenkinsci/jenkins/pull/2297#issuecomment-216710150
+                throw new IllegalStateException("Jenkins has not been started, or was already shut down");
+            }
+        }
+        return instance;
     }
 
     /**