servers: fix panicing when the Id doesn't exist

If the server id doesn't exist it's just panics. Check and return an appropriate error. The other changes are due to `gofmt`.
Hi @fatih  --  With my latest commit, err will not be nil if a server isn't found.  Can you provide a test case where this is false, along with a git sha of your version of Gophercloud and Perigee?  Thanks.