Companion: make `emitSuccess` and `emitError` private (#3832)

* remove redundant extraData from error

also make method private

* revert breaking change

* Update Uploader.js

packages/@uppy/companion/src/server/Uploader.js

@@ -293,7 +293,7 @@ class Uploader {
       const ret = await this.uploadStream(stream)
       if (!ret) return
       const { url, extraData } = ret
-      this.emitSuccess(url, extraData)
+      this.#emitSuccess(url, extraData)
     } catch (err) {
       if (err instanceof AbortError) {
         logger.error('Aborted upload', 'uploader.aborted', this.shortToken)
@@ -301,7 +301,7 @@ class Uploader {
       }
       // console.log(err)
       logger.error(err, 'uploader.error', this.shortToken)
-      this.emitError(err)
+      this.#emitError(err)
     } finally {
       emitter().removeAllListeners(`pause:${this.token}`)
       emitter().removeAllListeners(`resume:${this.token}`)
@@ -458,7 +458,7 @@ class Uploader {
    * @param {string} url
    * @param {object} extraData
    */
-  emitSuccess (url, extraData) {
+  #emitSuccess (url, extraData) {
     const emitData = {
       action: 'success',
       payload: { ...extraData, complete: true, url },
@@ -471,9 +471,10 @@ class Uploader {
    *
    * @param {Error} err
    */
-  emitError (err) {
+  #emitError (err) {
     // delete stack to avoid sending server info to client
-    // todo remove also extraData from serializedErr in next major
+    // todo remove also extraData from serializedErr in next major,
+    // see PR discussion https://github.com/transloadit/uppy/pull/3832
     const { stack, ...serializedErr } = serializeError(err)
     const dataToEmit = {
       action: 'error',