[Repo Command] Handle SystemCallError when creating "repos_dir"

Creating the "repos" directory when adding a new pod repo my fail due
to system-related errors. For example if the user doesn't have write
permissions to the ~/.cocoapods directory.
Handle the SystemCallError exception to fail gracefully by raising an
Informative exception.

Fixes #4177

lib/cocoapods/command/repo/add.rb

@@ -39,14 +39,19 @@ module Pod
         def run
           prefix = @shallow ? 'Creating shallow clone of' : 'Cloning'
           UI.section("#{prefix} spec repo `#{@name}` from `#{@url}`#{" (branch `#{@branch}`)" if @branch}") do
-            config.repos_dir.mkpath
-            Dir.chdir(config.repos_dir) do
-              command = ['clone', @url, @name]
-              command << '--depth=1' if @shallow
-              git!(command)
+            begin
+              config.repos_dir.mkpath
+            rescue SystemCallError => e
+              raise Informative, "Could not create repos directory '#{config.repos_dir}'\n#{e.errno.to_s} #{e.message}"
+            else
+              Dir.chdir(config.repos_dir) do
+                command = ['clone', @url, @name]
+                command << '--depth=1' if @shallow
+                git!(command)
+              end
+              Dir.chdir(dir) { git!('checkout', @branch) } if @branch
+              SourcesManager.check_version_information(dir)
             end
-            Dir.chdir(dir) { git!('checkout', @branch) } if @branch
-            SourcesManager.check_version_information(dir)
           end
         end
       end

spec/functional/command/repo/add_spec.rb

@@ -38,5 +38,14 @@ module Pod
         `git log --pretty=oneline`.strip.split("\n").size.should == 1
       end
     end
+
+    it 'raises an informative error when the repos directory fails to be created' do
+      repos_dir = config.repos_dir
+      def repos_dir.mkpath
+        raise SystemCallError, 'Operation not permitted'
+      end
+      e = lambda { run_command('repo', 'add', 'private', test_repo_path) }.should.raise Informative
+      e.message.should.match /Could not create repos directory '#{tmp_repos_path}'/
+    end
   end
 end