starlark::Error <-> buck2_error conversion

Summary:
Add implementations for buck2_error<->starlark. Code will be used in the next diff.

Note that there's `from_starlark` and `from_starlark_impl` because we need a way for caller to choose to not print stacktrace and/or be categorized (Though the categorized should eventually be removed once we categorize everything properly, but we will do this for now)

Reviewed By: JakobDegen

Differential Revision: D63487582

fbshipit-source-id: cdfa3ea9b8cd42a1d1428ff8420be8a96ee83a76

starlark_syntax/src/error.rs

@@ -119,6 +119,10 @@ impl Error {
         self.0.span()
     }
 
+    pub fn call_stack(&self) -> &CallStack {
+        self.0.call_stack()
+    }
+
     /// Set the span, unless it's already been set.
     pub fn set_span(&mut self, span: Span, codemap: &CodeMap) {
         self.0.set_span(span, codemap);