[dev/cli] ensure plugins/ and all watch source dirs exist (#78973)

Co-authored-by: spalger <spalger@users.noreply.github.com>

Author: Spencer

plugins/.empty

@@ -19,6 +19,7 @@
 
 import { resolve } from 'path';
 import { format as formatUrl } from 'url';
+import Fs from 'fs';
 
 import opn from 'opn';
 import { REPO_ROOT } from '@kbn/utils';
@@ -233,6 +234,14 @@ export class ClusterManager {
       )
     );
 
+    for (const watchPath of watchPaths) {
+      if (!Fs.existsSync(fromRoot(watchPath))) {
+        throw new Error(
+          `A watch directory [${watchPath}] does not exist, which will cause chokidar to fail. Either make sure the directory exists or remove it as a watch source in the ClusterManger`
+        );
+      }
+    }
+
     const ignorePaths = [
       /[\\\/](\..*|node_modules|bower_components|target|public|__[a-z0-9_]+__|coverage)([\\\/]|$)/,
       /\.test\.(js|tsx?)$/,