Raise an error on ambiguous refs

app/models/project.rb

@@ -1737,7 +1737,8 @@ class Project < ActiveRecord::Base
   end
 
   def protected_for?(ref)
-    return false if ref.nil? || repository.ambiguous_ref?(ref)
+    return false if ref.nil?
+    raise Repository::AmbiguousRefError if repository.ambiguous_ref?(ref)
 
     if Gitlab::Git.branch_ref?(ref) || Gitlab::Git.tag_ref?(ref)
       resolved_ref = ref

app/models/repository.rb

@@ -26,6 +26,7 @@ class Repository
   delegate :bundle_to_disk, to: :raw_repository
 
   CreateTreeError = Class.new(StandardError)
+  AmbiguousRefError = Class.new(StandardError)
 
   # Methods that cache data from the Git repository.
   #

spec/models/project_spec.rb

@@ -2583,8 +2583,8 @@ describe Project do
         project.repository.add_tag(project.creator, ref, 'master')
       end
 
-      it 'returns false' do
-        is_expected.to be_falsey
+      it 'raises an error' do
+        expect { subject }.to raise_error(Repository::AmbiguousRefError)
       end
     end