[nomaster] SI-7652 Bad tools fails loudly

A brief message is all that's required to
alleviate the look of consternation on the
face of your user.
[nomaster] SI-7652 REPL extended quest for tools

Javap tries harder and fails louder when
looking for tools.jar

This is a backport of the method as it
was first coded for partest.

If JAVA_HOME is wrong, it will either
fail with a message or succeed after
rooting about.

```
apm@mara:~/tmp/q$ JAVA_HOME=$JAVA7_HOME JAVACMD='/usr/lib/jvm/java-6-openjdk-amd64/bin/java' /home/apm/projects/snytt/build/pack/bin/scala
Welcome to Scala version 2.10.3-20130829-123337-59d6568daa (OpenJDK 64-Bit Server VM, Java 1.6.0_27).
Type in expressions to have them evaluated.
Type :help for more information.

scala> :javap
:javap [-lcsvp] [path1 path2 ...]

scala> :javap java.lang.Object
Failed: Could not load javap tool. Check that JAVA_HOME is correct.

apm@mara:~/tmp/q$ JAVA_HOME=/usr/lib/jvm/java-6-openjdk-amd64/jre JAVACMD='/usr/lib/jvm/java-6-openjdk-amd64/bin/java' /home/apm/projects/snytt/build/pack/bin/scala
Welcome to Scala version 2.10.3-20130829-123337-59d6568daa (OpenJDK 64-Bit Server VM, Java 1.6.0_27).
Type in expressions to have them evaluated.
Type :help for more information.

scala>

scala> :javap
:javap [-lcsvp] [path1 path2 ...]

scala> :javap java.lang.Object
Compiled from "Object.java"
```
Merge pull request #2895 from som-snytt/issue/7652-tools-jar-backport

SI-7652 REPL tools jar backport