You can create a sub process to handle your web service calls. See the following process:
I created a workitem handler for web service calling. The result of the web service call will set in a process variable. If the call fails, the process starts a user task and choose: retry, skip or abort. The user choise will set in an other process variable and evaluate in a gateway.
I've implemented a mechanisms that gives you retries, async executions and also allows human interventions.
You can take a look at the code here:
I will be working on improve what is there hopefully during the following month, but I believe that this project is exactly what you are looking for.