RFR: 8165736: Error message should be shown when JVMTI agent cannot be attached

[Yasumasa Suenaga]

Hi all,

If we try to attach invalid JVMTI agent via JVMTI.agent_load dcmd, we
will get "Command executed successfully". However, it implies error in
JVMTIAgentLoadDCmd.
This message is from JCmd.java when jcmd does not receive any output
from target VM.

I think HotSopt/jcmd should return useful error message to users to
understand the cause of failure.

I uploaded webrev for this issue. Could you review it?

  http://cr.openjdk.java.net/~ysuenaga/JDK-8165736/webrev.00/


This change is work fine on Fedora 26 x86_64 as following  jtreg testcases:

  - hotspot/jtreg/serviceability/attach
  - hotspot/jtreg/serviceability/dcmd/jvmti
  - jdk/com/sun/tools/attach

I cannot access JPRT. So I need a sponsor.
(I cannot test it on other platforms.)


Thanks,

Yasumasa