Add handling a new error code - 91

[dimonleksin]

Go version 1.20
Kafka version 3.7.1(latest)
IF call DescribeUserScramCredentials with non-existent user - kafka returned error code 91 and description "Attempt to describe a user credential that does not exist: "
In a Kafka Protocol Specification this not written, but in my practice this true.

[puellanivis]

https://github.com/apache/kafka/blob/trunk/core/src/main/scala/kafka/server/ZkAdminManager.scala#L805

The error does not seem to be attached to “Scram” specifically.

The code is itself just “resource not found” https://fossies.org/dox/kafka-3.7.1-src/enumorg_1_1apache_1_1kafka_1_1common_1_1protocol_1_1Errors.html#a0232a0a06a6b03b6868b74862c6393e3

[dimonleksin]

Yes, code named by "resource not found", but used this error only with users. So, i can rename this in next commit?

[puellanivis]

Whatever this name is when merged becomes the name that it has to be to keep from now until we can make a breaking change.

So, ideally, we want this error to be descriptive but also generic enough to cover all of its usage. And breaking with the common format and concurrence with Errors.RESOURCE_NOT_FOUND would violate the principle least astonishment.

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