Make SetConfig return an error

[astrieanna]

When Duffle calls SetConfig, it gets no feedback to relay to the user as to whether the settings were successfully applied. Duffle should be able to fail if the settings were not valid.

This is the relevant line to change the Configurable interface, but it would also need to change in each of the drivers.

[radu-matei]

There seems to be minimal disruption in both Duffle and Porter with this change:

• Duffle - https://github.com/deislabs/duffle/blob/c364840933b1886eddc01462090ea0494fc6bcfb/cmd/duffle/main.go#L127

• Porter - https://github.com/deislabs/porter/blob/1e971d7b0548ed44e798852ade1639acb6c12615/pkg/cnab/provider/runtime.go#L39

[carolynvs]

We'd be fine with that.