Fix Bug #22494 Instance Manager: Inconsistent error message wording

Use better wording

mysql-test/r/im_life_cycle.result

@@ -25,7 +25,7 @@ Success: the process has been stopped.
 -- 1.1.4.
 --------------------------------------------------------------------
 START INSTANCE mysqld3;
-ERROR HY000: Bad instance name. Check that the instance with such a name exists
+ERROR HY000: Unknown instance name
 START INSTANCE mysqld1;
 ERROR HY000: The instance is already started
 
@@ -33,7 +33,7 @@ ERROR HY000: The instance is already started
 -- 1.1.5.
 --------------------------------------------------------------------
 STOP INSTANCE mysqld3;
-ERROR HY000: Bad instance name. Check that the instance with such a name exists
+ERROR HY000: Unknown instance name
 
 --------------------------------------------------------------------
 -- 1.1.6.

server-tools/instance-manager/messages.cc

@@ -45,7 +45,7 @@ static const char *mysqld_error_message(unsigned sql_errno)
            " corresponds to your MySQL Instance Manager version for the right"
            " syntax to use";
   case ER_BAD_INSTANCE_NAME:
-    return "Bad instance name. Check that the instance with such a name exists";
+    return "Unknown instance name";
   case ER_INSTANCE_IS_NOT_STARTED:
     return "Cannot stop instance. Perhaps the instance is not started, or was"
            " started manually, so IM cannot find the pidfile.";