Ensure npm scripts (via Gulp) emit errors

Whilst we suppress non-zero exit codes for `gulp dev` we should still exit if anything writes to `stderr`

tasks/run.mjs

@@ -14,8 +14,13 @@ export async function npmScript (name, args = []) {
   return new Promise((resolve, reject) => {
     const script = spawn(command, ['run', name, '--silent', ...args])
 
+    // Send output to console
     script.stdout.on('data', (data) => console.log(data.toString()))
-    script.stderr.on('data', (data) => console.error(data.toString()))
+
+    // Emit errors to error listener
+    script.stderr.on('data', (data) => {
+      script.emit('error', new Error(data.toString()))
+    })
 
     // Reject on actual script errors to exit `gulp dev`
     script.on('error', reject)