[SourceManager] Disable remote check on update in favour of a warning

Closes https://github.com/CocoaPods/CocoaPods/issues/2388

lib/cocoapods/sources_manager.rb

@@ -158,8 +158,10 @@ module Pod
                 output = git!("pull --ff-only")
                 UI.puts output if show_output && !config.verbose?
               rescue Informative => e
-                raise Informative, 'An error occurred while performing ' \
-                  "`git pull` on repo `#{source.name}`.\n" + e.message
+                UI.warn "CocoaPods was not able to update the " \
+                  "`#{source.name}` repo. If this is an unexpected issue " \
+                  "and persists you can inspect it running " \
+                  "`pod repo update --verbose`"
               end
             end
             check_version_information(source.data_provider.repo)
@@ -334,8 +336,7 @@ module Pod
       #
       def git_sources
         aggregate.all.select do |source|
-          git_repo?(source.data_provider.repo) &&
-            git_remote_reachable?(source.data_provider.repo)
+          git_repo?(source.data_provider.repo)
         end
       end
     end

spec/unit/sources_manager_spec.rb

@@ -124,6 +124,16 @@ module Pod
         SourcesManager.update(test_repo_path.basename.to_s, true)
       end
 
+      it "prints a warning if the update failed" do
+        UI.warnings = ''
+        set_up_test_repo_for_update
+        Dir.chdir(test_repo_path) do
+          `git remote set-url origin https://example.com`
+        end
+        SourcesManager.update(test_repo_path.basename.to_s, true)
+        UI.warnings.should.include('not able to update the `master` repo')
+      end
+
       it 'returns whether a source has a reachable git remote' do
         SourcesManager.git_remote_reachable?(repo_make('a_new_repo_that_is_new')).should.be.false
         SourcesManager.git_remote_reachable?(SourcesManager.master_repo_dir).should.be.true