Improve error feedback for duplicate deploy keys (#13112)

Instead of a generic HTTP 500 error page, a flash message is rendered with the deploy key page template to inform the user that a key with the intended title already exists. 

Fixes #13110

models/error.go

@@ -547,7 +547,7 @@ func IsErrDeployKeyNameAlreadyUsed(err error) bool {
 }
 
 func (err ErrDeployKeyNameAlreadyUsed) Error() string {
-	return fmt.Sprintf("public key already exists [repo_id: %d, name: %s]", err.RepoID, err.Name)
+	return fmt.Sprintf("public key with name already exists [repo_id: %d, name: %s]", err.RepoID, err.Name)
 }
 
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