Fix invite code bug

[IscoRuta98]

When an invalid code is used, there is no error message for the user. This PR fixes this issue by adding validation error message.

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[codecov-commenter]

Codecov Report

Merging #553 (75f59f4) into main (56bdf1c) will increase coverage by 0.77%. The diff coverage is 0.00%.

:mega: This organization is not using Codecov’s GitHub App Integration. We recommend you install it so Codecov can continue to function properly for your repositories. Learn more

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   46.38%   47.16%   +0.77%     
==========================================
  Files         173      173              
  Lines        2796     2801       +5     
  Branches      534      537       +3     
==========================================
+ Hits         1297     1321      +24     
+ Misses       1499     1480      -19     

Impacted Files	Coverage Δ
web-client/src/app/services/invite.service.ts	11.11% <0.00%> (-0.89%)	:arrow_down:
web-client/src/app/views/register/register.page.ts	41.42% <0.00%> (-1.86%)	:arrow_down:

... and 2 files with indirect coverage changes