by Gabriel Kulevicius.
I had the same issue, ugrading from moodle 2.0.x to 2.1.x, and as I saw, the problem was that the database was inconsistent, as it looks like it was a database result from previous upgrade tries with error results (not success).
The database had temporary created tables that broke the upgrade and didnt let me continue.
I could success the upgrade with:
1- upgrading to installing the latest 2.1.x installation (in my case 2.1.9)
2. Deleting some tables (previous backup)
drop table mdl_question_usages;
drop table mdl_quiz_reports;
drop table mdl_question_attempt_steps;
drop table mdl_question_attempt_step_data;
drop table mdl_question_hints;
Now, the functional user is testing the results ...
good luck! (to me also)