Add custom error for gitaly unavailable

What does this MR do and why?

Adding custom error when gitaly is down

Screenshots or screen recordings

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

1.go to any project 1.type gdk stop gitaly in terminal 1.refresh page and observe error This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383737 (closed)