endpoint /mentorship_relation/send_request does not validate users id

**Describe the bug**
When mentor_id to mentee_id is invalid Internal server error occurs (500)

**To Reproduce**
Steps to reproduce the behaviour:
1. Go to localhost try out'/mentorship_relation/send_request.'
2. Send mentorship relation request with non existing ` "mentor_id": 0`, or `"mentee_id": 0`.
3. See internal server error

**Expected behavior**
Backend should return bad request with information that user with specific id does not exists

**Additional context**
`AttributeError: 'tuple' object has no attribute 'name'`