Show rest of task page when graph errors out

Summary: T15772.

Test Plan: Add a throw statement in graph calculation, load a task that has a graph, see message.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25570