fix: error and pointer for unverified user buying free tickets

[shreyanshdwivedi]

Fixes #6394

Short description of what this resolves:

Wrong error and pointer for unverified user

Changes proposed in this pull request:

• changes pointer from order to user
• changes error to ForbiddenException and changes error message

{
  "errors": [{
     "status": 403, 
     "source": {
          "pointer": "/data/relationships/user", 
          "code": "unverified-user"
      }, 
      "title": "Access Forbidden", 
      "detail": "Unverified user cannot place free orders"
   }], 
   "jsonapi": {"version": "1.0"}
}

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[codecov]

Codecov Report

Merging #6395 into development will increase coverage by 0.02%.
The diff coverage is 0%.

@@               Coverage Diff               @@
##           development    #6395      +/-   ##
===============================================
+ Coverage        65.16%   65.19%   +0.02%     
===============================================
  Files              287      287              
  Lines            14810    14810              
===============================================
+ Hits              9651     9655       +4     
+ Misses            5159     5155       -4

Impacted Files	Coverage Δ
app/api/orders.py	27.91% <0%> (ø)	⬆️
app/api/helpers/scheduled_jobs.py	24.03% <0%> (+3.1%)	⬆️

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