Added custom error messages functionality for individual fields.
Changed $error_msg to $errors
Fixed a missed variable $errors in set_rules().
Removed extra space between closing parenthesis.
Updated change log message for form validation custrom errors.
Updated comment messages.
Added spaces between parameters and fixed the arrays to be more readable.
Merge branch 'develop' into feature/form_validation/custom_error_per_field
Added the missing "is" in the comment
Changed the rest of 'error_msg' to 'errors'
Added space after 'if' condition
Fixed coding styles in form_validation.rst
Fixed the comment and array alignment.
Merge pull request #2804 from abdmaster/feature/form_validation/custom_error_per_field

Added custom error messages functionality for individual fields.