Check the exit code from the server process

Reviewed-by: NMatt Caswell <matt@openssl.org>
Reviewed-by: NRichard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2205)

util/TLSProxy/Proxy.pm

@@ -316,6 +316,7 @@ sub clientstart
         print "Waiting for server process to close: "
               .$self->serverpid."\n";
         waitpid( $self->serverpid, 0);
+        die "exit code $? from server process\n" if $? != 0;
     }
     return 1;
 }