Graceful degradation of the Branches Page when Gitaly is unavailable

If Gitaly is not available, add an empty state page with a message, for example "The Gitaly server is not available at this time" ?

• Don't issue a 500 if Gitaly is not available
• Display a message on the relevant page

Related to #332339 (closed).

Proposal

• Add a non-dismissable In-page alert.
• Alert title should read: Unable to load branches
• Alert message should read The git server, Gitaly, is not available at this time. Contact your administrator.