[keyserver] Better document MySQL errno

Summary: Using variable names here makes the code self-documenting. Initial discussion here, full reference of MySQL 5.7 error codes here.

Test Plan: Honestly I haven't tested this, but I have looked at git diff very carefully and I think it's safe

Reviewers: palys-swm, atul

Reviewed By: palys-swm

Subscribers: Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4157