avoid t.Fatal() in tests (#9189)

I've been burned a few times by tests that simply fatal, requiring me to
run another build to learn more about what the mismatch was. Avoid this.

This is part of a long running goal of mine to make testing "better".

cmd/mattermost/commands/roles_test.go

@@ -17,7 +17,7 @@ func TestAssignRole(t *testing.T) {
 	CheckCommand(t, "roles", "system_admin", th.BasicUser.Email)
 
 	if result := <-th.App.Srv.Store.User().GetByEmail(th.BasicUser.Email); result.Err != nil {
-		t.Fatal()
+		t.Fatal(result.Err)
 	} else {
 		user := result.Data.(*model.User)
 		if user.Roles != "system_user system_admin" {
@@ -28,7 +28,7 @@ func TestAssignRole(t *testing.T) {
 	CheckCommand(t, "roles", "member", th.BasicUser.Email)
 
 	if result := <-th.App.Srv.Store.User().GetByEmail(th.BasicUser.Email); result.Err != nil {
-		t.Fatal()
+		t.Fatal(result.Err)
 	} else {
 		user := result.Data.(*model.User)
 		if user.Roles != "system_user" {

cmd/mattermost/commands/user_test.go

@@ -50,12 +50,10 @@ func TestCreateUserWithoutTeam(t *testing.T) {
 	CheckCommand(t, "user", "create", "--email", email, "--password", "mypassword1", "--username", username)
 
 	if result := <-th.App.Srv.Store.User().GetByEmail(email); result.Err != nil {
-		t.Fatal()
+		t.Fatal(result.Err)
 	} else {
 		user := result.Data.(*model.User)
-		if user.Email != email {
-			t.Fatal()
-		}
+		require.Equal(t, email, user.Email)
 	}
 }
 
@@ -92,7 +90,7 @@ func TestChangeUserEmail(t *testing.T) {
 		t.Fatal("should've updated to the new email")
 	}
 	if result := <-th.App.Srv.Store.User().GetByEmail(newEmail); result.Err != nil {
-		t.Fatal()
+		t.Fatal(result.Err)
 	} else {
 		user := result.Data.(*model.User)
 		if user.Email != newEmail {