Better handling of finding users to merge

* When searching on idnumber, multiple user records can be returned. So using MUST_EXIST parameter for Moodle database API. This also ensures that a user record exists. Removed now unnecessary error handling and related strings.
* Handling if trying to merge the same user accounts.
* Fixed debugging messages that appear in Moodle 2.5, because setType wasn't being called on the correct form field. Also, changed type from PARAM_INT to PARAM_USERNAME.
* Replaced now deprecated call to get system context.
* Updated lang strings for new error string and removed error strings no longer referenced.