* 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
* Consistent license message for all the go files
* Fixing the last set of unconsistencies with the license headers
* Addressing PR review comments
* Fixing busy.go and busy_test.go license header
