Add invalid document error handling

[jonnyom]

Why?

We should be catching and handling this error more gracefully.

[SeanHealy33]

Nice

[sidonath]

We started getting undefined method '[]' for nil:NilClass at lib/intercom/request.rb:114 after upgrading to 3.7.5 and it seems as this PR could be addressing the problem.

However, it's hard to be certain as I have no idea what hides between nil, since line 113 is a little bit misleading, as it doesn't always raise in case of a failure:

raise_errors_on_failure(response) if parsed_body.nil?

Maybe raise_errors_on_failure should raise for every response with code >= 400 or at least log the error somehow?

[jonnyom]

Hey @sidonath I think this PR wasn't actually added in 3.7.5, but rather 3.7.6

Sorry for the delay in getting back to you, this fell through the cracks.

[sidonath]

Hi @jonnyom! I've since realized that the issue was unrelated to these changes and opened #467