improve http status codes

[dantownsend]

Some of the HTTP status codes were incorrect - the endpoints were returning 401, which means permission denied, when 422 is more appropriate, as they are validation errors.

This is important for Piccolo Admin, as we configure axios to intercept any 401 errors, and redirect to the login page.

[dantownsend]

@sinisaos We need this change, because that's why in this PR it was redirecting to the login page if there was an error in the change password form:

piccolo-orm/piccolo_admin#168

[codecov-commenter]

Codecov Report

Merging #161 (8e193ba) into master (96a8a26) will not change coverage.
The diff coverage is 75.00%.

@@           Coverage Diff           @@
##           master     #161   +/-   ##
=======================================
  Coverage   88.38%   88.38%           
=======================================
  Files          29       29           
  Lines        1576     1576           
=======================================
  Hits         1393     1393           
  Misses        183      183           

Impacted Files	Coverage Δ
piccolo_api/register/endpoints.py	80.43% <ø> (ø)
piccolo_api/session_auth/endpoints.py	75.15% <0.00%> (ø)
piccolo_api/change_password/endpoints.py	85.71% <100.00%> (ø)

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[sinisaos]

@dantownsend This is great and should work.

[dantownsend]

@sinisaos Cool - I'll release this, then will release the change password feature in Piccolo Admin.