add UART errors for embedded-io

[robamu]

Related issue: #577

[chrysn]

This looks good to me if the intention around the error type is to have details perfectly suited to specific transports (that is not clear to me from its docs; those only refer to the corresponding std type).

If that is not the intention, would more abstract errors work? Overrun fits that pattern, but "FrameFormat" could be a more generic "ConfigurationMismatch" error ("In the communication with the data source/sink, the peer behaved in a way that indicates that the peers are configured differently."), "UnreliableConnection" ("The received data was internally erroneous in a way that indicates that the underlying connection is unreliable, eg. due to failing CRC or parity checks"). "Noise" can fall in either category, but then again, not all UARTs distinguish those in the first place, and it might suffice to lump them in with UnreliableConnection.