ChangeQueryBuilder: Improve error message when is:mergeable is not supported

The "check configs" part of the message is useless for the majority of
users who are unlikely to have access to the server's configuration, so
drop that. Reword the message to be more consistent with the ones when
the 'is:private' 'is:started' and 'is:wip' operators are not supported.

Change-Id: I93ad6ca19134bb7f80e2e263126419ef14484533
diff --git a/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java b/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
index 47f5ad6..df5d291 100644
--- a/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
+++ b/java/com/google/gerrit/server/query/change/ChangeQueryBuilder.java
@@ -579,7 +579,7 @@
 
     if ("mergeable".equalsIgnoreCase(value)) {
       if (!args.indexMergeable) {
-        throw new QueryParseException("server does not support 'mergeable'. check configs");
+        throw new QueryParseException("'is:mergeable' operator is not supported by server");
       }
       return new BooleanPredicate(ChangeField.MERGEABLE);
     }
diff --git a/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java b/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
index 962b691..9c12052 100644
--- a/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
+++ b/javatests/com/google/gerrit/server/query/change/AbstractQueryChangesTest.java
@@ -3093,7 +3093,7 @@
     assertThat(thrown.getCause()).isInstanceOf(QueryParseException.class);
     assertThat(thrown)
         .hasMessageThat()
-        .contains("server does not support 'mergeable'. check configs");
+        .contains("'is:mergeable' operator is not supported by server");
   }
 
   protected ChangeInserter newChange(TestRepository<Repo> repo) throws Exception {