Change some error tests to use assert_raises

[brandur]

Previously, we had quite a few error tests that were written like this:

begin
  client.execute_request(:post, "/v1/charges")
rescue Stripe::InvalidRequestError => e
  assert_equal(404, e.http_status)
  assert_equal(true, e.json_body.is_a?(Hash))
end

The trouble with that pattern is that although it'll usually work,
the test will incorrectly pass if no error at all is thrown because the
rescue never activates and therefore the assertions never run.

We change them to use assert_raises like so:

e = assert_raises Stripe::InvalidRequestError do
  client.execute_request(:post, "/v1/charges")
end
assert_equal(404, e.http_status)
assert_equal(true, e.json_body.is_a?(Hash))

The weird part is that many of the tests were already using
assert_raises, so here we're just converting them all over to use the
same convention.

I've also made a few whitespace tweaks. None of them are significant,
but they were an attempt to standardize a little on the whitespace
layout of many of these tests which were similar.

r? @ob-stripe

[brandur-stripe]

TY!