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