This issue was brought up a couple of years ago, but not adequately handled. It needs to be re-opened.
In short, when I try to upload a .csv file to Moodle, if there is a user in the .csv file whose identification does not appear in Moodle, then Moodle will halt and catch fire. In particular, it gives the error message:
User mapping error: Could not find user with idnumber of ***.
Import failed. No data was imported.
This is not an adequate handling of the error condition. I have several classes combined into one account, and this has 200 students. If Moodle finds a single student whose ID cannot be confirmed, Moodle will do nothing. I have to find the appropriate ID, remove it from the .csv file, save the .csv file, and start again from the beginning with the "grade import" page, matching up the fields, and so on. If there is another student who cannot be identified, I have to repeat the procedure again.
A better way of handling the problem is to have Moodle provide a complete list of students whose grades could not be uploaded. If there are not too many of these, Moodle should upload the rest of the data (and inform the user that this has been done); otherwise, Moodle should give the user the option of uploading the data that can be found.