Use descriptive error type for SASL authentication failures

[NyaliaLui]

Currently, RP brokers throw an exception of type std::runtime_error when an unexpected SASL authentication failure occurs.
https://github.com/vectorizedio/redpanda/blob/bf5953e45f7d59a4755874047e6bc6726d3cde9e/src/v/kafka/server/requests.cc#L199

A recent slack conversation discussed that type std::runtime_error reads as if the exception was not handled gracefully.

Instead, use a error type that better reflects the situation for SASL authentication failures in the kafka sub-system.