StoreResult support class needed for Repository

As the source says, "a dirt-simple little value object".

RSpec: 857 examples, 0 failures (but see Issue #87)
RuboCop: 168 files inspected, no offenses detected
Added ErrorFactory code and specs.

RSpec: 860 examples, 0 failures (but see Issue #87)
RuboCop: 170 files inspected, no offenses detected