Inadvertent omission from ErrorFactory spec.

RSpec: 861 examples, 0 failures (but see Issue #87)
RuboCop: 170 files inspected, no offenses detected
First stab at UserRepository code and specs.

This covers initialisation and the `#add` public method, as well as
handing back correct `StoreResult` instances for success and failure.

Also corrected the `ErrorFactory` code and specs, and added
`be_saved_entity_for` custom RSpec matcher.

RSpec: 866 examples, 0 failures (but see Issue #87)
RuboCop: 174 files inspected, no offenses detected