Update SendGrid Adapter to return ok/error tuple

[germsvel]

Requires: #571

What changed?

We update the SendGridAdapter to abide by the new Adapter behaviour. Adapters must now returns an {:ok, email} or {:error, error}.

Note on implementation with throw/catch

Building SendGrid's body requires a lot of deeply nested data manipulation. Some code that is deeply nested within branching logic raises errors.

Rather than returning an error tuple at a really low level and have to bubble up the error all the way until it's returned, we opted for a simpler approach: throwing and catching the errors.

We throw and catch {:error, error} when we used to raise error.

It's possible we could do something different in the future, like bubbling up the errors and returning them without having to resort to throw and catch, but right now we want to minimize the number of changes for these raises deep in the call stack.

Raising (not returning) configuration errors

While working on this, an important question came up. Should we raise or return an error because of a missing API key? Currently we raise.

I think it's okay to raise an error in that case because it's a configuration issue. It's not the same type of error that happens when you can't build an email, deliver it, or even get a response that isn't 200. That error is a configuration issue and so the sooner we notify the user of Bamboo, the better.

We can also think about it this way: we want to return an ok/error tuple because we people want to handle email building or delivery issues. Having an API key missing isn't that kind of issue. It means the library is configured incorrectly, so it seems good to raise an error.

[lady3bean]

Circle is failing on mix test with:

== Compilation error in file lib/bamboo/adapters/send_grid_adapter.ex ==
** (CompileError) lib/bamboo/adapters/send_grid_adapter.ex:60: undefined function build_api_error/3
    (elixir) src/elixir_locals.erl:107: :elixir_locals."-ensure_no_undefined_local/3-lc$^0/1-0-"/2
    (elixir) src/elixir_locals.erl:108: anonymous fn/3 in :elixir_locals.ensure_no_undefined_local/3
    (stdlib) erl_eval.erl:680: :erl_eval.do_apply/6

Otherwise changes lgtm

[germsvel]

Thanks @lady3bean. The build is failing because this branch depends on #571 which introduced the build_api_error function. Once I merge 571, and rebase this one, the CI error should go away.