better concurrency debugging, --concurrency flag for npx quartz build

quartz/trace.ts

@@ -1,17 +1,22 @@
 import chalk from "chalk"
 import process from "process"
+import { isMainThread } from "workerpool"
 
 const rootFile = /.*at file:/
 export function trace(msg: string, err: Error) {
   const stack = err.stack
-  console.log()
-  console.log(
+
+  const lines: string[] = []
+
+  lines.push("")
+  lines.push(
     "\n" +
-      chalk.bgRed.black.bold(" ERROR ") +
-      "\n" +
-      chalk.red(` ${msg}`) +
-      (err.message.length > 0 ? `: ${err.message}` : ""),
+    chalk.bgRed.black.bold(" ERROR ") +
+    "\n" +
+    chalk.red(` ${msg}`) +
+    (err.message.length > 0 ? `: ${err.message}` : ""),
   )
+
   if (!stack) {
     return
   }
@@ -23,11 +28,20 @@ export function trace(msg: string, err: Error) {
     }
 
     if (!line.includes("node_modules")) {
-      console.log(` ${line}`)
+      lines.push(` ${line}`)
       if (rootFile.test(line)) {
         reachedEndOfLegibleTrace = true
       }
     }
   }
-  process.exit(1)
+
+  const traceMsg = lines.join("\n")
+  if (!isMainThread) {
+    // gather lines and throw
+    throw new Error(traceMsg)
+  } else {
+    // print and exit
+    console.error(traceMsg)
+    process.exit(1)
+  }
 }