Handling of failure during GattClient operations is missing

The current GattClient API does not provide any way to catch a failure during a GattClient procedure. The following procedure miss this feature:
- Primary service discovery
- Characteristic discovery
- read attribute value
- write attribute value

With the current implementation, if something fail during one of this procedure, the client will never now that the procedure has ended and will not be able to react to such failures.

Start by locating the GattClient API and its primary service discovery, characteristic discovery, read attribute value, and write attribute value procedures. Trace how each procedure currently signals completion, then determine how failures should be exposed to callers; done means all four procedures let clients detect and react to failure.

Written by the indexing model from the issue text.