Errors in tests stops execution.

In 1.2.0 the behavior was to throw an exception on error. This resulted in a stacktrace pointing to the plugin (confusing behavior). I removed it for version 1.2.1 but forgot that it will cause the runner to continue after a failed test.

This change makes it trigger a system.exit(1) upon failure.
Handling different exceptions based on type of exception.

A parsing error (stuff like: x is not defined) will now print a complete javascript stacktrace.

If there for some reason is a java exception thrown (test that a using the rhino java environment for example java.io.File or similar) this error will also show a complete stack trace.

If there is a regular javascript exception only the error message will be shown. I could not find any way to actually print a complete stacktrace to that error due to a rhino bug. So if you do any stuff like this in you javascript:

    throw "an error";

This will not have a stacktrace.
Update to readme and build.sbt version.
Added a code comment for the issue with no stack trace for regualar javascript exceptions.
Backing out a change that isn't backwards compatible with sbt 0.12 cross-compilation
Setting to SNAPSHOT version, and bumping Scala/sbt versions
Put back an exception for test failure, but this time a TestsFailedException from sbt to elicit consistent behavior
Documenting 1.2.2