don't fail all results on 1 invalid manifest

Created by: sqs

fix #1445

Fixes the case where an extension has invalid JSON that fails to unmarshal into the Go type. For example, the following JSON used to cause the entire extension list to fail, but now the error is shown only on the single bad extension:

  "activationEvents": [
    {"language": "foo"}
  ],

(Note that {"language": "foo"} is not a valid entry for that; it should be "onLanguage:foo".)

Also only returns valid extension JSON from the API.