Check and remove duplicate strings related to connection errors

[ihalaij1]

The below was mentioned in PR #1185:

There are multiple, a little different error messages for the grading connection errors in the code base. Some of them could be maybe removed and replaced with the almost identical ones. It is not immediately obvious why there are multiple similar strings (in django.po). Some of them are used in similar situations (based on a quick glance), some in a clearly different scenario. Since most of them are old, we should check if they are really up-to-date. Some of them promise that the course staff is emailed about the errors, but I am not sure if the email is really sent in all cases.

Similar strings include:

• SUBMISSION_ERROR_ALERT_COMMUNICATION_ERROR_W_SERVICE
• EXERCISE_ERROR_COMMUNICATION
• EXERCISE_SERVICE_ERROR_CONNECTION_FAILED
• ASSESSMENT_SERVICE_ERROR_CONNECTION_FAILED
• ERROR_ALERT_EXERCISE_ASSESSMENT_SERVICE_MALFUNCTIONING