ATTError cannot support additional common error codes

[dpogueproton]

The ATTError enum does not include (and cannot be extended to include) additional common profile and service error codes, as per the spec: https://www.bluetooth.com/wp-content/uploads/Files/Specification/HTML/CSS_v12/out/en/supplement-to-the-bluetooth-core-specification/common-profile-and-service-error-codes.html

In particular, because peripheral.willWrite expects a block that returns an ATTError?, it's not possible to return any of these additional error codes (or application-specific error codes).

[dpogueproton]

Per https://www.bluetooth.com/wp-content/uploads/Files/Specification/HTML/Core-54/out/en/host/attribute-protocol--att-.html#UUID-eefd3e8d-9b16-3af8-1fb4-fa90f52262e8_informaltable-idm13358908515990 values between 0x80 – 0x9F are also reserved for application-level error codes, so a mechanism to extend ATTError with application-specific values is needed.

[colemancda]

We can change ATTError to a struct, or just add the enum cases. What stack are you using that's doing to work properly with custom ATT Errors? CoreBluetooth certainly is not going to support it.

[dpogueproton]

I see some examples on GitHub where CoreBluetooth's CBATTError.Code is extended with additional options, such as https://github.com/tidepool-org/BluetoothCommonKit/blob/dev/Sources/BluetoothCommonKit/Extension/CBATTError.swift and https://github.com/tidepool-org/LoopDeviceSimulator/blob/dev/DeviceSimulator/Extension/CBATTError.swift, which is similar to what we'd be hoping to accomplish with ATTError

[colemancda]

In that case just open a PR change the ATTError enum to a RawRepresentable struct.