moodle-tool_mergeusers - #41 : strings revisited

Related to issue #41.

* Revisited lang strings to show specific messages when merge fails,
  according to whether transactions are supported.
* Updated renderer to update lang keys.
* Deleted an incorrect phpdoc.
moodle-tool_mergeusers - #41 : revisited code

Related to issue #41.

* Revisited code to simplify it.
Merge pull request #42 from jpahullo/issue41

moodle-tool_mergeusers - #41 : strings revisited