bake: avoid nesting error diagnostics

With changes to the lazy evaluation, the evaluation order is no longer
fixed - this means that we can follow long and confusing paths to get to
an error.

Because of the co-recursive nature of the lazy evaluation, we need to
take special care that the original HCL diagnostics are not discarded
and are preserved so that the original source of the error can be
detected. Preserving the full trace is not necessary, and probably not
useful to the user - all of the file that is not lazily loaded will be
eagerly loaded after all struct blocks are loaded - so the error would
be found regardless.

Signed-off-by: Justin Chadwell <me@jedevc.com>

bake/hclparser/expr.go

@@ -14,15 +14,7 @@ func funcCalls(exp hcl.Expression) ([]string, hcl.Diagnostics) {
 	if !ok {
 		fns, err := jsonFuncCallsRecursive(exp)
 		if err != nil {
-			return nil, hcl.Diagnostics{
-				&hcl.Diagnostic{
-					Severity: hcl.DiagError,
-					Summary:  "Invalid expression",
-					Detail:   err.Error(),
-					Subject:  exp.Range().Ptr(),
-					Context:  exp.Range().Ptr(),
-				},
-			}
+			return nil, wrapErrorDiagnostic("Invalid expression", err, exp.Range().Ptr(), exp.Range().Ptr())
 		}
 		return fns, nil
 	}