Define error codes and reuse

[malinthaprasan]

Description

Refactor all errors in platform-api/src/internal/constants/error.go

eg:

type APIError struct {
    Code    string
    Message string
}

func (e *APIError) Error() string {
    return e.Message
}

// Define as variables
var (
    ErrAPIIdentifierExists = &APIError{Code: "API_IDENTIFIER_EXISTS", Message: "API identifier already exists"}
    ErrAPINameVersionExists = &APIError{Code: "API_NAME_VERSION_EXISTS", Message: "API name and version combination already exists"}
)