RFR: JDK-8180709: java -javaagent:agent.jar with run-time that does not contain java.instrument prints confusing error

[Gary Adams]

Here's a simple fix to include additional information when the 
instrument library is
missing from the jre and the command line requests -javaagent.

  Issue: https://bugs.openjdk.java.net/browse/JDK-8180709
  Webrev: http://cr.openjdk.java.net/~gadams/8180709/webrev.00/