Improve blob fixture spec error handling

This change improve the error handling in spec/frontend/fixtures/raw.rb
so when a single spec fails it does not disrupt other specs such as
`NoMethodError` caused by a nil value.

spec/frontend/fixtures/raw.rb

@@ -7,41 +7,45 @@ RSpec.describe 'Raw files', '(JavaScript fixtures)' do
 
   let(:namespace) { create(:namespace, name: 'frontend-fixtures' )}
   let(:project) { create(:project, :repository, namespace: namespace, path: 'raw-project') }
-  let(:response) { @blob.data.force_encoding('UTF-8') }
+  let(:response) { @response }
+
+  def blob_at(commit, path)
+    @response = project.repository.blob_at(commit, path).data.force_encoding('UTF-8')
+  end
 
   after do
     remove_repository(project)
   end
 
   it 'blob/notebook/basic.json' do
-    @blob = project.repository.blob_at('6d85bb69', 'files/ipython/basic.ipynb')
+    blob_at('6d85bb69', 'files/ipython/basic.ipynb')
   end
 
   it 'blob/notebook/markdown-table.json' do
-    @blob = project.repository.blob_at('f6b7a707', 'files/ipython/markdown-table.ipynb')
+    blob_at('f6b7a707', 'files/ipython/markdown-table.ipynb')
   end
 
   it 'blob/notebook/worksheets.json' do
-    @blob = project.repository.blob_at('6d85bb69', 'files/ipython/worksheets.ipynb')
+    blob_at('6d85bb69', 'files/ipython/worksheets.ipynb')
   end
 
   it 'blob/notebook/math.json' do
-    @blob = project.repository.blob_at('93ee732', 'files/ipython/math.ipynb')
+    blob_at('93ee732', 'files/ipython/math.ipynb')
   end
 
   it 'blob/pdf/test.pdf' do
-    @blob = project.repository.blob_at('e774ebd33', 'files/pdf/test.pdf')
+    blob_at('e774ebd33', 'files/pdf/test.pdf')
   end
 
   it 'blob/text/README.md' do
-    @blob = project.repository.blob_at('e774ebd33', 'README.md')
+    blob_at('e774ebd33', 'README.md')
   end
 
   it 'blob/images/logo-white.png' do
-    @blob = project.repository.blob_at('e774ebd33', 'files/images/logo-white.png')
+    blob_at('e774ebd33', 'files/images/logo-white.png')
   end
 
   it 'blob/binary/Gemfile.zip' do
-    @blob = project.repository.blob_at('e774ebd33', 'Gemfile.zip')
+    blob_at('e774ebd33', 'Gemfile.zip')
   end
 end