Fix error from invalid sha for `include` files

`External::File::Local` uses the sha from the context when fetching
local include files. If that sha is not valid, an exception is raised by
Gitaly.

This commit fixes that error by catching the Gitaly exception and
returning an appropriate error.

Changelog: fixed

lib/gitlab/ci/config/external/file/local.rb

@@ -33,6 +33,10 @@ module Gitlab
 
             def fetch_local_content
               context.project.repository.blob_data_at(context.sha, location)
+            rescue GRPC::InvalidArgument
+              errors.push("Sha #{context.sha} is not valid!")
+
+              nil
             end
 
             override :expand_context_attrs

spec/lib/gitlab/ci/config/external/file/local_spec.rb

@@ -81,6 +81,16 @@ RSpec.describe Gitlab::Ci::Config::External::File::Local do
         expect(local_file.valid?).to be_falsy
       end
     end
+
+    context 'when the given sha is not valid' do
+      let(:location) { '/lib/gitlab/ci/templates/existent-file.yml' }
+      let(:sha) { ':' }
+
+      it 'returns false and adds an error message stating that included file does not exist' do
+        expect(local_file).not_to be_valid
+        expect(local_file.errors).to include("Sha #{sha} is not valid!")
+      end
+    end
   end
 
   describe '#content' do