Client fails when singularity image needs to be built

I just tried using the BmiClientSingularity with a docker image like so:

`self.bmi = BmiClientSingularity(image="docker://ewatercycle/pcrg-grpc4bmi:setters", ...)`

but it failed with a `FutureTimeoutError`, I think because the initial build is taking so long. It would be good if we could catch this, and we need to decide whether we want it to fail or whether it should complete the build but print a nice message (like suggested in #95)