More UserRepository error checking: record not found -- and more

Also:
* Added UserRepository#update;
* Implemented what I hope is the final-for-now fix to ErrorFactory's
.create method (assuming that the input quacks sufficiently like an
ActiveModel::Errors object), along with a long description that, if
you're reading this, you probably want to give ErrorFactory a fresh read
too;
* Added UserRepository#find_by_slug; handy to stub for testing;
* Added :be_an_error_has_for custom matcher, for testing ErrorFactory.

RSpec: 872 examples, 0 failures (but see Issue #87)
RuboCop: 175 files inspected, no offenses detected