Handle serialErrors

Summary:
Solve T9485 by

• SerialLayer: emit serialErrors
• AtCore: handleErrors from serialLayer
• TestClient: Improve change to disconnect state

Most serial Errors are shown. Unless you get the error for device removal in that case, you will get a serial error about not being able to read from the device Then after some time about 2 minutes for my test device you will be disconnected from the device and atcore will close the serial layer.

Reviewers: patrickelectric, tcanabrava, laysrodrigues, leandrosantiago

Reviewed By: patrickelectric, laysrodrigues

Subscribers: Atelier: AtCore

Tags: Atelier: AtCore

Differential Revision: https://phabricator.kde.org/D15080