Proper stack traces

[sbarzowski]

[sbarzowski]

First step: #59

[sbarzowski]

A few things left to do:

1. Add some useful and easily available information, like object names (https://github.com/google/go-jsonnet/blob/master/parser/context.go#L326).
2. [UX] if the stack trace is very long and the middle part gets elided, it should tell the user how to increase the stack trace size limit.
3. Avoid thunk from <thunk from <thunk from. If we cannot show something useful (see 1), then we should at least be concise and clear.

Later we could also try to reduce noise from the internals.

[sbarzowski]

@anguslees @sparkprime Any other suggestions?

[anguslees]

(oh duh, I should increase the stack trace size limit :P I somehow forgot that go-jsonnet probably had the same tunables as c++ jsonnet ..)

That list sounds good to me!