- Remove Error.Delegate
- Skip JVM stack trace handling
- Consistent formatting of raw delegate errors and regular positioned errors

Instead of sprinkling error propagation with or without stack frames all over the Evaluator and other parts of the codebase, we now do it in one place (visitExpr) with additional logic in Error.withStackFrame. Duplicate frames are skipped, an error without any stack frame always gets the current position, and all other frames are added or skipped depending on the Expr at the position.

This makes stack traces much more descriptive. We also change some of the reported positions / expr types to better match the semantics of the errors.

They are reported as sjsonnet.ParseError with normal stack traces instead of the previous one-liners which required customized error messages for parse errors in imported files. Errors are now stored as Error objects instead of Strings in the parse cache. This paves the way for error reporting during static evaluation and it is already required now to translate the paths in parse error stack traces to a different base directory (when reusing a cached parse error from a previous run).

Unknown variables and illegal self/super/$ references are now checked statically as they are supposed to be. Error messages are consistent with Google Jsonnet's.
Fixes databricks#122