More pleasantly handle broken or missing git repositories (#17747)

* More pleasantly handle broken or missing git repositories

In #17742 it was noted that there a completely invalid git repository underlying a
repo on gitea.com. This happened due to a problem during a migration however, it
is not beyond the realms of possibility that a corruption could occur to another
user.

This PR adds a check to RepoAssignment that will detect if a repository loading has
failed due to an absent git repository. It will then show a page suggesting the user
contacts the administrator or deletes the repository.

Fix #17742

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

* Update options/locale/locale_en-US.ini

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

models/repo.go

@@ -146,6 +146,7 @@ const (
 	RepositoryReady           RepositoryStatus = iota // a normal repository
 	RepositoryBeingMigrated                           // repository is migrating
 	RepositoryPendingTransfer                         // repository pending in ownership transfer state
+	RepositoryBroken                                  // repository is in a permanently broken state
 )
 
 // TrustModelType defines the types of trust model for this repository
@@ -289,6 +290,11 @@ func (repo *Repository) IsBeingCreated() bool {
 	return repo.IsBeingMigrated()
 }
 
+// IsBroken indicates that repository is broken
+func (repo *Repository) IsBroken() bool {
+	return repo.Status == RepositoryBroken
+}
+
 // AfterLoad is invoked from XORM after setting the values of all fields of this object.
 func (repo *Repository) AfterLoad() {
 	// FIXME: use models migration to solve all at once.