[Fix] if onerror handler is not set, rethrow the error

Author: SunriseFox

README.md

@@ -677,6 +677,7 @@ The IMAP client has several events you can attach to by setting a listener
 ### Handling fatal error event
 
 The invocation of `onerror` indicates an irrecoverable error. When `onerror` is fired, the connection is already closed, hence there's no need for further cleanup.
+If no `onerror` is specified, the error will be thrown.
 
 ## Get your hands dirty
 

dist/client.js

@@ -105,7 +105,11 @@ export default class Client {
     clearTimeout(this._idleTimeout)
 
     // propagate the error upwards
-    this.onerror && this.onerror(err)
+    if (this.onerror) {
+      this.onerror(err)
+    } else {
+      throw err
+    }
   }
 
   //