enhance(build): handle missing BLOG_ROOT + log rg errors (#9340)

The blogs repo is not public so `BLOG_ROOT` environment variable 
has not been advertised. If the variable is not set then `yarn tool spas` 
command throws an error which doesn't make sense at all.

Co-authored-by: Claas Augner <[email protected]>

build/utils.ts

@@ -311,6 +311,11 @@ export function makeTOC(doc) {
 }
 
 export function findPostFileBySlug(slug: string): string | null {
+  if (!BLOG_ROOT) {
+    console.warn("'BLOG_ROOT' not set in .env file");
+    return null;
+  }
+
   try {
     const { stdout, stderr, status } = spawnSync(rgPath, [
       "-il",
@@ -320,8 +325,12 @@ export function findPostFileBySlug(slug: string): string | null {
     if (status === 0) {
       const file = stdout.toString("utf-8").split("\n")[0];
       return file;
+    }
+    const message = stderr.toString();
+    if (message) {
+      console.error(`error running rg: ${message}`);
     } else {
-      console.error(`error running rg: ${stderr}`);
+      console.error(`Post ${slug} not found in ${BLOG_ROOT}`);
     }
   } catch {
     console.error("rg failed");