This change should implement the changes we discussed in #30 regarding requiring a worker URL or Registration in the constructor.
I've added a constant for error messages so they are easier to handle in the tests with changes, although still not easily shareable by just importing a file because the push-client is transpiled the tests in the browser are not (so no easy way I can think of to require in a file) - Thoughts on this would be super helpful.
Fixes #30
@wibblymat @jeffposnick @petele
This change should implement the changes we discussed in #30 regarding requiring a worker URL or Registration in the constructor.
I've added a constant for error messages so they are easier to handle in the tests with changes, although still not easily shareable by just importing a file because the push-client is transpiled the tests in the browser are not (so no easy way I can think of to require in a file) - Thoughts on this would be super helpful.