Fix target value in htmx:targetError when inheritance has been used

[scrhartley]

Description

When hx-target is inherited, currently the value of event.detail.target for htmx:targetError is null.
This value should instead be the value that htmx was using to try to resolve the target.

Testing

Added a test for the event when inheritance is not being used, since that seemed to not exist yet.
Then added a test which uses an inherited value for hx-target.
(I also manually tested that the event doesn't fire when the inherited target does exist.)

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded