Raise an error when given a known bad path.

There are a few cases where a bad path (`nil`, the empty string, and a
string that doesn’t start with `/`) could cause the client to make a
nonsense request to the arbiter. Rather than make this request and rely
on the URL arbiter to fail, we can add checks in the client to make sure
that all the requests make sense.
Raise `ArgumentError` instead of a client error.

The reasoning behind this is that errors inheriting from
`GOVUK::Client::Errors::BaseError` represent something going wrong in
the API itself (be that a validation error, server error or network
problem). For cases of programmer error (or, if you prefer, violations
of a method's preconditions), we raise the appropriate standard Ruby
error.
Merge pull request #10 from alphagov/check-bad-paths

Raise an error when given a known bad path.