make service handling of non-error-not-founds explicit

repository/account/interface.go

@@ -5,6 +5,8 @@ import "arimelody-web/model"
 type AccountRepository interface {
     GetAll() ([]*model.Account, error)
     GetCount() (int, error)
+    // Fetches an account by ID, returning an error if one was encountered.
+    // If the account does not exist, both response fields are nil.
     GetByID(id string) (*model.Account, error)
     GetByUsername(username string) (*model.Account, error)
     GetByEmail(email string) (*model.Account, error)

service/account/account.go

@@ -4,6 +4,7 @@ import (
 	"arimelody-web/model"
 	repository "arimelody-web/repository/account"
 	"errors"
+	"fmt"
 	"log"
 )
 
@@ -28,15 +29,33 @@ func (s *AccountService) GetCount() (int, error) {
 }
 
 func (s *AccountService) GetByID(id string) (*model.Account, error) {
-    return s.repo.GetByID(id)
+    if account, err := s.repo.GetByID(id); err != nil {
+        return nil, err
+    } else if account == nil {
+        return nil, fmt.Errorf("Account does not exist: %s", id)
+    } else {
+        return account, nil
+    }
 }
 
 func (s *AccountService) GetByUsername(username string) (*model.Account, error) {
-    return s.repo.GetByUsername(username)
+    if account, err := s.repo.GetByUsername(username); err != nil {
+        return nil, err
+    } else if account == nil {
+        return nil, fmt.Errorf("Account does not exist: %s", username)
+    } else {
+        return account, nil
+    }
 }
 
 func (s *AccountService) GetByEmail(email string) (*model.Account, error) {
-    return s.repo.GetByEmail(email)
+    if account, err := s.repo.GetByEmail(email); err != nil {
+        return nil, err
+    } else if account == nil {
+        return nil, fmt.Errorf("Account does not exist with email: %s", email)
+    } else {
+        return account, nil
+    }
 }
 
 func (s *AccountService) Create(