Issues with the acceptGitLabMR step

## Issue

### Context
- **Gitlab plugin version**: 1.5.11
- **Gitlab version**: 10.7.2-ce
- **Jenkins version**: 2.89.4
- **Job type**: Pipeline

### Problem description

I'm having a couple of problems when I run the step for accepting a MR

```groovy
acceptGitLabMR(useMRDescription: true, removeSourceBranch: true)
```
The first problem is that the pipeline is actually running although the ```Ignore WIP Merge Requests ``` check is marked in the Jenkins job. I'm triggering the job when a comment takes place in the MR. I don't know if the ```Ignore WIP Merge Requests ``` option is supposed to work in my case?

Another problem is that when the ```acceptGitLabMR``` step fails, it only logs the error so the pipeline continues. I think it would be better to throw an exception to give the user the opportunity to catch it and decide wether the job should fail or continue.

One last issue is that in the case that the branch is actually merged, it is not deleted although the ```removeSourceBranch``` parameter is set as ```true```.