Make "JIRA Issues" field work better with noncredentialed accounts

Summary:
Currently, users get an error when making any changes to this field if they don't have a linked JIRA account.

Instead:

  - We should only raise an error if they're trying to //add// issues, and only on the new issues. It's always fine to remove issues, and existing issues the author can't see are also fine.
  - When we can't add things because there's no account (vs because there's a permissions error or they don't exist), raise a more tailored exception.

Test Plan:
  - As JIRA and non-JIRA users, made various edits to this field.
  - Got appropriate exceptions, including better tailoring.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: mbishopim3, epriestley

Differential Revision: https://secure.phabricator.com/D8676