storage.exceptions: use value property to format all storage exceptions

[aesteve-rh]

#218 introduces a value property that shadows the property from the base class in _HoldingLVMCommandError:

    @property
    def value(self):
        return ", ".join(f"{k}={v}" for k, v in self.__dict__.items() if v)

We can use a similar strategy for all storage errors (e.g., in StorageException), and avoid formatting when the exception is constructed. This will give a more consistent look along all storage exceptions and will avoid discrepancies in the same exception when called from different places.