REST API - Create diff comment thread - fix misleading error message

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Summary

My dyslexia cost me a lot of time here.

When we try to submit a comment on an MR diff and we specify incorrect file name, instead of getting a message about non-existent file, we get a message about a missing optional line_code attribute.

This misleads the API user and makes them (me) look for a problem in the wrong place.

Steps to reproduce

1. Sample project: viktomas/empty-test-2!1 (diffs)

2. Get the MR version so you can craft the create message request.

   curl -H "Authorization: Bearer $TOKEN" 'https://gitlab.com/api/v4/projects/22433161/merge_requests/1/versions' | jq

   [
     {
       "id": 124423305,
       "head_commit_sha": "943df554d4cc69a63ad43acc9f28bf6a8fa99e48",
       "base_commit_sha": "b62a77777f42dd3bdd0c8f6dcf190ee4581f88a3",
       "start_commit_sha": "b62a77777f42dd3bdd0c8f6dcf190ee4581f88a3",
       "created_at": "2020-11-13T14:29:00.136Z",
       "merge_request_id": 77972283,
       "state": "collected",
       "real_size": "1"
     }
   ]

3. Try to create an MR message, but misspell the file name (the actual filename is tets-selection.js)

   curl  --request POST --header "Authorization: Bearer $TOKEN" --form "body=Via API" \
   --form "position[old_path]=test-selection.js" --form "position[new_path]=test-selection.js" \
   --form "position[old_line]=20" \
   --form "position[head_sha]=943df554d4cc69a63ad43acc9f28bf6a8fa99e48" \
   --form "position[base_sha]=b62a77777f42dd3bdd0c8f6dcf190ee4581f88a3" \
   --form "position[start_sha]=b62a77777f42dd3bdd0c8f6dcf190ee4581f88a3" \
   --form "position[position_type]=text" \
   https://gitlab.com/api/v4/projects/22433161/merge_requests/1/discussions

   {
       "message": "400 (Bad request) \"Note {:line_code=\u003e[\"can't be blank\", \"must be a valid line code\"]}\" not given"
   }

Example Project

viktomas/empty-test-2!1 (diffs)

What is the current bug behavior?

The error message says that there is a missing line_code attribute.

What is the expected correct behavior?

The error message says that there is no such file.

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes