check test helper shouldn't silently swallow errors

[six-shopify]

What are you adding in this PR?

Another small one from me. Currently the check() test helper (which wraps the main check() export) will happily run a broken rule and report no issues. That is, given a check like this:

export const SomeWonkyCheck: LiquidCheckDefinition = {
  // metadata block excluded...
  create(context) {
    return {
      async LiquidRawTag(node) {
        throw new Error("I tried to do something weird!")
      },
    };
  },
};

a test like this will be green:

it('does not report an error, even if the rule implementation is busted', async () => {
  const theme = {
    'sections/example.liquid': toLiquid(someConfigThatTriggersAnEdgeCase),
  };

  const offenses = await check(theme, [SomeWonkyCheck]);

  expect(offenses).toHaveLength(0);
});

This PR overrides the default no-op onError so that we properly rethrow errors when running tests, rather than silently squelching all error output. Fortunately, this doesn't seem to unmask any broken test cases. I can't really think of any reason this shouldn't be the default — callers of the check() test helper can wrap it in try...catch if they genuinely have some reason to expect a check to throw, though my intuition is that checks should never be doing that on purpose.

[six-shopify]

SourceCodeType is already the default for this generic param (our shared ESLint config reports this but it's not set up for this repo yet, see #781)

[charlespwd]

LGTM! Doesn't need a changeset since this doesn't change any public API

Also wow TIL.