[MM-37557] Move error out of client4 response (#18101)

* Return an error seperately from Response

* Remove BuildErrorResponse

* Drop Response.Error from model/client4.go

* Migrate require.Nil checks

* Migrate require.NotNil checks

* More manual fixes

* Move error check out of CheckOKStatus and CheckCreatedStatus

* Move error check out of CheckForbiddenStatus

* Move error check out of CheckUnauthorizedStatus

* Move error check out of CheckNotFoundStatus

* Move error check out of CheckBadRequestStatus

* Move error check out of CheckNotImplementedStatus and CheckRequestEntityTooLargeStatus

* Move error check out of CheckInternalErrorStatus

* Move error check out of CheckServiceUnavailableStatus

* Remove error check from checkHTTPStatus

* Remove remaining references to Response.Error

* Check previously unchecked errors

* Manually fix compile and linter errors

* Return error in CreateWebSocket methods

* Return error instead of *AppError in DoApi methods

* Manually fix bad replacments

* Conistently return Response and error

* Use err instead of seperate bool return value to indicate success

* Reduce ussage of model.AppError in web/oauth_test.go

* Remove client4.Must

* Check error in buf.ReadFrom

* Fix failing tests

model/client4_test.go

@@ -67,7 +67,8 @@ func TestClient4CreatePost(t *testing.T) {
 	}))
 
 	client := NewAPIv4Client(server.URL)
-	_, resp := client.CreatePost(post)
+	_, resp, err := client.CreatePost(post)
+	assert.NoError(t, err)
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 }
 
@@ -89,7 +90,8 @@ func TestClient4SetToken(t *testing.T) {
 	client := NewAPIv4Client(server.URL)
 	client.SetToken(expected)
 
-	_, resp := client.GetMe("")
+	_, resp, err := client.GetMe("")
+	assert.NoError(t, err)
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 }
 
@@ -111,6 +113,7 @@ func TestClient4MockSession(t *testing.T) {
 	client := NewAPIv4Client(server.URL)
 	client.MockSession(expected)
 
-	_, resp := client.GetMe("")
+	_, resp, err := client.GetMe("")
+	assert.NoError(t, err)
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 }