fix non reporting of failures with floating IP assignment

It seems that commit essex-1-1003-g9728ae5
inadvertently removed a 'raise', thus hiding
all processutils.ProcessExecutionError exceptions
apart from the one indicating "Interface not found".

Fixes bug: 1178658
Change-Id: I2bcf8c3a19ebebc81a38ce6bf7b54065d62834c0