Transport-specific associated error types

[AlfioEmanueleFresta]

TransportError is not currently ideal for transport-specific errors. It has generic overly generic variants, and does not include details of the specific underlying cause.

Includes:

• Replaces TransportError with an associated error type to Channel. For example, CableError.
• Associated types should support CtapError and PlatformError, eg. implementing From<CtapError> and From<PlatformError>.
• Replace generic variants (eg. ConnectionLost) in favour of more specific root causes, eg. CableError::HandshakeFailed(snow::Error).
• CableUxUpdate::Error to use CableError, instead of TransportError.