fix(core/pipeline): handle missing trigger fields in importDeliveryConfig stage

[erikmunson]

Currently when there's an issue with the trigger used for the importDeliveryConfig stage and some fields are missing, we throw trying to call substring() on the hash field and the entire executions view goes blank (along with permanently busting the entire UI until a full page reload).

This change uses a Not found fallback for all the trigger-related fields we surface, so things render and the page continues to work. Looks like this:

[luispollo]

Thanks a bunch for fixing this, @erikmunson!

Maybe we could go back to something like this for the validation to avoid this kind of problem:
6190531

[erikmunson]

my 2 cents: if something about the trigger is misconfigured, I'd expect it to show up as a validation error on the trigger. In the open source world the only way to run into this problem is if you misconfigure your Git trigger (i.e. don't add the required fields), so in that world you'd already be seeing a validation error while configuring.

The reason we never saw any config-time validation internally at Netflix is because our internal source code trigger is actually a source code trigger plus and artifact trigger plus a build trigger, so when you leave out critical bits of config for a git/source code trigger we look at it and go 'well, you configured this artifact trigger correctly, so I guess you're good to go'.

I'd rather look at that trigger validation problem as the one to solve (probably by splitting apart our internal trigger) since that problem applies not just to this stage, but to anything regarding validation (for all 3 kinds of triggers).