[resultdb] Add more context to error messages

... on the off chance that somebody has the `LUCI_CONTEXT` env var set
locally with some invalid value.

Change-Id: I38c247b42706529e01de649ed3a0f47a62e3cd6e
Reviewed-on: https://fuchsia-review.googlesource.com/c/shac-project/shac/+/927714
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Anthony Fandrianto <atyfto@google.com>
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
diff --git a/internal/reporting/resultdb.go b/internal/reporting/resultdb.go
index 40ce6cb..9dfd5ec 100644
--- a/internal/reporting/resultdb.go
+++ b/internal/reporting/resultdb.go
@@ -248,13 +248,14 @@
 
 // resultSinkCtx returns the rdb stream port if available.
 func resultSinkCtx() (*resultSinkContext, error) {
-	b, err := os.ReadFile(os.Getenv("LUCI_CONTEXT"))
+	path := os.Getenv("LUCI_CONTEXT")
+	b, err := os.ReadFile(path)
 	if err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to load LUCI_CONTEXT=%q: %w", path, err)
 	}
 	var ctx luciContext
 	if err = json.Unmarshal(b, &ctx); err != nil {
-		return nil, err
+		return nil, fmt.Errorf("failed to load LUCI_CONTEXT=%q: %w", path, err)
 	}
 	// We are clearly running inside a LUCI_CONTEXT luciexe environment but rdb
 	// stream was not started. Hard fail since it means we need to fix the recipe.