IGNITE-23946 Improve clarity of 'Cache is already indexed' exception message

modules/core/src/main/java/org/apache/ignite/internal/processors/query/schema/SchemaOperationException.java

@@ -142,7 +142,7 @@ private static String message(int code, String objName) {
                 return "Index already exists: " + objName;
 
             case CODE_CACHE_ALREADY_INDEXED:
-                return "Cache is already indexed: " + objName;
+                return "Cache is already indexed (already supports SQL): " + objName;
 
             case CODE_VIEW_NOT_FOUND:
                 return "View doesn't exist: " + objName;

modules/indexing/src/test/java/org/apache/ignite/internal/processors/cache/index/DynamicEnableIndexingConcurrentSelfTest.java

@@ -462,7 +462,7 @@ public void testConcurrentEnableIndexing() throws Exception {
                         SchemaOperationException opEx = e.getCause(SchemaOperationException.class);
 
                         assertEquals(SchemaOperationException.CODE_CACHE_ALREADY_INDEXED, opEx.code());
-                        assertEquals("Cache is already indexed: " + POI_CACHE_NAME, opEx.getMessage());
+                        assertEquals("Cache is already indexed (already supports SQL): " + POI_CACHE_NAME, opEx.getMessage());
                     }
 
                     return null;

modules/indexing/src/test/java/org/apache/ignite/internal/processors/cache/index/H2DynamicTableSelfTest.java

@@ -356,7 +356,7 @@ public void testCreateTableOnExistingCache() {
                 execute(client(), String.format(createTemplate, "NameTest1", cacheName));
 
                 return null;
-            }, IgniteSQLException.class, "Cache is already indexed: " + cacheName);
+            }, IgniteSQLException.class, "Cache is already indexed (already supports SQL): " + cacheName);
         }
         finally {
             client().destroyCache("new");