Restore propagation of ErrDependenciesLeft (#18325)

Unfortunately #17643 prevented all propagation of ErrDependenciesLeft meaning
that dependency errors that prevent closing of issues get swallowed.

This PR restores propagation of the error but instead swallows the error in the
places where it needs to be swallowed.

Fix #18223

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/pull/merge.go

@@ -105,7 +105,10 @@ func Merge(pr *models.PullRequest, doer *user_model.User, baseGitRepo *git.Repos
 		close := ref.RefAction == references.XRefActionCloses
 		if close != ref.Issue.IsClosed {
 			if err = issue_service.ChangeStatus(ref.Issue, doer, close); err != nil {
-				return err
+				// Allow ErrDependenciesLeft
+				if !models.IsErrDependenciesLeft(err) {
+					return err
+				}
 			}
 		}
 	}