ldap: an error is logged when a user is not an admin

[junkb]

• Gitea version (or commit ref): 1.4.0
• Git version: n/a
• Operating system: ubuntu 17.10
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist: https://gist.github.com/junkb/b5abbf7ec4ee1f975a6a423b87c51d37

Description

when the ldap search is performed to determine if a user is an admin, if there is no result [meaning the user in question is not an admin], an error is logged. this is a little misleading - if there is no result, because the user is not in the admin group, then things are working exactly as they should, and there is no error. i think it would be helpful if this outcome were handled and logged differently than an error