improve debugging ux

Currently the debugging ux is pretty bad.
Actions with invalid JS in template expressions silently fail, tracing action calls is not possible, seeing the call-stack now that lambdas are available is not possible.
Forgetting to `return` a value from a exprssion with multiple js expressions fails silently.

Console.log is available for use within expression templates but is very cumbersome to use.

I've had an initial look at j2v8 usage within jasonette but it doesn't seem to expose errors as exceptions thrown into Java. It does appear to have a [Debug API available](https://github.com/eclipsesource/J2V8/blob/c97922e95c3ba7d6e544f81bcf6bf14b788e03a0/src/main/java/com/eclipsesource/v8/debug/DebugHandler.java) but it does not appear to be documented.

Start by reviewing Jasonette's existing J2V8 usage and the linked J2V8 DebugHandler API, since no source files are named. Define how invalid template JavaScript, missing returns, action tracing, lambda call stacks, and console logging should be surfaced, then verify that the agreed debugging workflow reports these cases clearly.

Written by the indexing model from the issue text.