internal/worker: make json parsing errors obvious.

We directly propagate the err from json.Unmarshall without any context.
This makes it difficult to understand that the error message is even
coming from a json parsing issue.

Change-Id: Iec8a04b48092a079c2b44faf4cbd39cd5ac81ae9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/702495
Auto-Submit: Markus Kusano <kusano@google.com>
Reviewed-by: Ethan Lee <ethanalee@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/worker/analysis.go

diff --git a/internal/worker/analysis.go b/internal/worker/analysis.go
index d115f5f..ef78440 100644
--- a/internal/worker/analysis.go
+++ b/internal/worker/analysis.go
@@ -308,7 +308,7 @@
 	}
 	var tree analysis.JSONTree
 	if err := json.Unmarshal(out, &tree); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("Unable to parse analyzer's -json diagnostics: %w", err)
 	}
 	return tree, nil
 }