Add error handling to GattClient procedures

With the current API their is no way to know if an error occur during the following GattClient procedures:
- `read`: If a read procedure fail, it should be possible to report this information to the application.
- `write`: If a write procedure fail, it should be possible to report this information to the application.

Start by locating the GattClient read and write procedure entry points and reviewing how their results currently reach the application. Define what failure information must be exposed for each procedure, then verify that both failed reads and failed writes can be distinguished from successful operations.

Written by the indexing model from the issue text.