chore(workers/repository): add more information to lookup error warnings (renovatebot#42120)

When a `lookupError` occurs, it's currently unclear as to why the lookup
error occurred.

It seems to be more common in the Go ecosystem where a lookup error
occurs, and Renovate autocloses PRs, only to recreate them in a
future run.

To provide more useful debugging information to users, we can add the
actual `lookupError` string itself.

Author: jamietanna

lib/workers/repository/process/lookup/index.ts

@@ -171,7 +171,7 @@ export async function lookupUpdates(
         // If dependency lookup fails then warn and return
         const warning: ValidationMessage = {
           topic: config.packageName,
-          message: `Failed to look up ${config.datasource} package ${config.packageName}`,
+          message: `Failed to look up ${config.datasource} package ${config.packageName}: ${lookupError}`,
         };
         logger.debug(
           {