[FEATURE] Handle sql.ServiceError() as error interface

[rariyama]

Thank you for the gread library. This is my first issue so I'm sorry if this issue is missing the point in advance.

Problem Statement
When executing sql via StatementExecutionAPI, we'll get the execution status as sql.StatementStatus.
If both API and SQL execution succeed, one of the values like SUCCEEDED is assigned to State and Error becomes nil wheares If SQL execution fails, some of the values like error reasons will be assigned to sql.ServiceError
This struct is related to error, then it'll be more useful to handle the struct as error interface. For example, it'll be possible to compare via errors.As() method if it can be handled as error interface.

Proposed Solution
Add Error() function to meet the requirements of error interface.

Additional Context
I'll cooperate about this task as much as possible.