Improve recovery from panel action rendering exceptions, and mark "Changeset" queries as not suitable for panel generation

Summary:
Fixes T13443. When a panel raises an exception during edit action generation, it currently escapes to top level. Instead, catch it more narrowly.

Additionally, mark "ChangesetSearchEngine" as not being a suitable search engine for use in query panels. There's no list view or search URI so it can't generate a sensible panel.

Test Plan:

• Added a changeset panel to a dashboard.
• Before: entire dashboard fataled.
• After: panel fataled narrowly, menu fatals narrowly, dashboard no longer permits creation of another Changeset query panel.

Maniphest Tasks: T13443

Differential Revision: https://secure.phabricator.com/D20902