Proposal: Define Error Variables

[uji]

I’d like to allow library users to distinguish user caused errors and library or rpc caused errors.
This allows for better error handling by the user.

For example, in go-sql-driver/mysql, errors are defined by variables.
https://github.com/go-sql-driver/mysql/blob/ad9fa14acdcf7d0533e7fbe58728f3d216213ade/errors.go#L18-L31

In addition, complex ones are expressed by defining dedicated types.
https://github.com/go-sql-driver/mysql/blob/ad9fa14acdcf7d0533e7fbe58728f3d216213ade/errors.go#L57-L62