Handle failed promises with better messaging

[gmarcosb]

If a promise fails & returns a null value, fail with some additional info

[nknapp]

Could you specify in more detail the input, the helpers and the expected result? That would help me understand what you want to do exactly.

You could also do this by adding a test case to your PR:

• You probably need a helper that returns a rejecting promise here
• And a new testcase here

[gmarcosb]

So this is simply meant to be better logging

Effectively, today if a promise returns a null, the whole thing blows up without clear messaging about what failed ( replacePlaceholdersRecursivelyIn blows up with null inputs here)

This change makes it so that when a promise returns a null value, instead of blowing up in replacePlaceholdersRecursivelyIn with a null access, we blow up before with some information about the promise that returned a null value

In other words, it fails both before & after this change, but after this change it fails with a more helpful message vs no member toHtml on undefined

Additional test: I'm not too famliar with nodejs, mostly just hobbled this together by copy-pasting; there doesn't seem to be any tests for the failure cases, only the happy cases - shouldn't there already be some tests that cover this? 😅 Would happily modify it as that's more straightforward