More user friendly error reporting for privileges
More user friendly error reporting for privileges

Related Issue: https://github.com/github/gh-ost/issues/136

New output:
```
2016-08-05 11:39:07 DEBUG Privileges: Super: true, REPLICATION SLAVE: false, ALL on *.*: false, ALL on `sakila`.*: true
2016-08-05 11:39:07 ERROR User has insufficient privileges for migration. Needed: SUPER, REPLICATION SLAVE and ALL on `sakila`.*
2016-08-05 11:39:07 FATAL 2016-08-05 11:39:07 ERROR User has insufficient privileges for migration. Needed: SUPER, REPLICATION SLAVE and ALL on `sakila`.*
```

wfxiang08/gh-ost,shuhaowu/gh-ost,wfxiang08/gh-ost,druud62/gh-ost,groenewold-pythian/gh-ost,groenewold-pythian/gh-ost,shuhaowu/gh-ost,druud62/gh-ost