Make some perl scripts usable with git bisect run

For that they need to return between 0 and 124 on error, while die returns
255, causing bisect-run to abort.
diff --git a/tests/scripts/curves.pl b/tests/scripts/curves.pl
index f4942c6..b7cfdf6 100755
--- a/tests/scripts/curves.pl
+++ b/tests/scripts/curves.pl
@@ -36,14 +36,16 @@
 system( "cp $config_h $config_h.bak" ) and die;
 sub abort {
     system( "mv $config_h.bak $config_h" ) and warn "$config_h not restored\n";
-    die $_[0];
+    warn $_[0];
+    exit 1;
 }
 
 for my $curve (@curves) {
     system( "cp $config_h.bak $config_h" ) and die "$config_h not restored\n";
+    system( "make clean" ) and die;
+
     # depends on a specific curve. Also, ignore error if it wasn't enabled
     system( "scripts/config.pl unset MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED" );
-    system( "make clean" ) and die;
 
     print "\n******************************************\n";
     print "* Testing without curve: $curve\n";