No longer bubble 'error' events from a-asset-items as these interfere with Karma

[mrxz]

Description:
I noticed that not all unit tests are being executed. Turns out that emitting error events causes Karma to stop executing further tests. Interestingly enough this does not seem to be tied to the error event handler in __init.test.js. This PR ensure that the error event emitted by <a-asset-item> doesn't bubble, effectively resolving the issue.

Ideally we should avoid using the error event name, but I figured that is a bigger breaking change than changing it to not bubble.

Before: ✔ 1771 tests completed / ℹ 12 tests skipped
After: ✔ 2475 tests completed / ℹ 16 tests skipped
Note: Above results are obtained by running npm run test, which counts test double between FF and Chrome

Changes proposed:

• No longer bubble error events from a-asset-items

[dmarcos]

Awesome catch! Thanks so much!