Do not exit early on partial commit search failure

Created by: camdencheek

Before this commit, we were using an errgroup, which cancels its context when any of its tasks fails. For commit search, we'd rather continue our search, but return the error. Currently, errors are not surfaced to the user if results are returned, so if there are more results than from the failing repo, the user won't see that results are incomplete. That is out of scope of this fix though.

Fixes https://github.com/sourcegraph/sourcegraph/issues/29474