improve error msg (#5010)

https://github.com/transloadit/uppy/pull/5003#issuecomment-2007045486

packages/@uppy/companion/src/config/companion.js

@@ -91,7 +91,7 @@ const validateConfig = (companionOptions) => {
   if (server && server.path) {
     // see https://github.com/transloadit/uppy/issues/4271
     // todo fix the code so we can allow `/`
-    if (server.path === '/') throw new Error('server.path cannot be set to /')
+    if (server.path === '/') throw new Error('If you want to use \'/\' as server.path, leave the \'path\' variable unset')
   }
 
   if (providerOptions) {