Perl test script: Avoid some aborts, which made the whole build/test process terminate.

mysql-test/lib/mtr_process.pl:
  Change from "mtr_error()" to "mtr_warning()" on some problems,
  because "error" makes the whole suite abort which then makes "Do-compile" terminate,
  so none of the following steps (including other etst suites) will be done.

mysql-test/lib/mtr_process.pl

@@ -454,8 +454,7 @@ sub mtr_kill_leftovers () {
 
         if ( kill(0, @pids) )           # Check if some left
         {
-          # FIXME maybe just mtr_warning() ?
-          mtr_error("can't kill process(es) " . join(" ", @pids));
+          mtr_warning("can't kill process(es) " . join(" ", @pids));
         }
       }
     }
@@ -468,7 +467,7 @@ sub mtr_kill_leftovers () {
   {
     if ( mtr_ping_mysqld_server($srv->{'port'}, $srv->{'sockfile'}) )
     {
-      mtr_error("can't kill old mysqld holding port $srv->{'port'}");
+      mtr_warning("can't kill old mysqld holding port $srv->{'port'}");
     }
   }
 }