Index start/activate command: fix unknown name use

Before this fix, using the IndexStartCommand [1] while passing an
invalid or unknown index name with the --force option led to an NPE in
the log, thus a fatal error in user's console. Also, using an unknown
index name either without --force or through the sibling
IndexActivateCommand [2] used to display an inaccurate console message.

This fix always shows a clearer message to the user, upon passing an
unknown index name to either [1] or [2]. It also now prevents that NPE.

[1] https://gerrit-review.googlesource.com/Documentation/cmd-index-start.html
[2] https://gerrit-review.googlesource.com/Documentation/cmd-index-activate.html

Bug: Issue 8715
Change-Id: I16cb800fbe9af00e47bb22105d1c4d4bf044fdc5

gerrit-server/src/main/java/com/google/gerrit/server/index/AbstractVersionManager.java

@@ -121,6 +121,16 @@ public abstract class AbstractVersionManager implements LifecycleListener {
     return false;
   }
 
+  /**
+   * Tells if an index with this name is currently known or not.
+   *
+   * @param name index name
+   * @return true if index is known and can be used, otherwise false.
+   */
+  public boolean isKnownIndex(String name) {
+    return defs.get(name) != null;
+  }
+
   protected <K, V, I extends Index<K, V>> void initIndex(
       IndexDefinition<K, V, I> def, GerritIndexStatus cfg) {
     TreeMap<Integer, Version<V>> versions = scanVersions(def, cfg);