fix: changed the error message from custom_auth serializer

[utkarsh-1905]

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Changes

Changed the error message of custom_auth serializer from "Invalid Email Address" to "Email already exists. Please log in."

Solves #3886
Please describe.

How did you test this code?

Manually

Please describe.

1. Run the api and frontend locally
2. Signup using an email and then logout and try to signup again.
3. The error message shows "Invalid email address" which is confusing to the user.

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 10, 2024 3:24pm

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.98%. Comparing base (132ef77) to head (c3d3b29).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3924   +/-   ##
=======================================
  Coverage   95.98%   95.98%           
=======================================
  Files        1135     1135           
  Lines       36182    36182           
=======================================
  Hits        34731    34731           
  Misses       1451     1451           

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