Show a proper error if gem path is not writable

Instead of showing the bug report place with an error at a randome
place.

bundler/lib/bundler/errors.rb

@@ -75,10 +75,22 @@ def action
       end
     end
 
+    def permission_type
+      case @permission_type
+      when :create
+        "executable permissions for all parent directories and write permissions for `#{parent_folder}`"
+      else
+        "#{@permission_type} permissions for that path"
+      end
+    end
+
+    def parent_folder
+      File.dirname(@path)
+    end
+
     def message
       "There was an error while trying to #{action} `#{@path}`. " \
-      "It is likely that you need to grant #{@permission_type} permissions " \
-      "for that path."
+      "It is likely that you need to grant #{permission_type}."
     end
 
     status_code(23)

bundler/lib/bundler/rubygems_gem_installer.rb

@@ -19,7 +19,9 @@ def install
       FileUtils.rm_rf gem_dir
       FileUtils.rm_rf spec.extension_dir
 
-      FileUtils.mkdir_p gem_dir, :mode => 0o755
+      SharedHelpers.filesystem_access(gem_dir, :create) do
+        FileUtils.mkdir_p gem_dir, :mode => 0o755
+      end
 
       extract_files
 

bundler/spec/commands/install_spec.rb

@@ -678,6 +678,36 @@ def run
     end
   end
 
+  describe "when bundle gems path does not have write access", :permissions do
+    let(:gems_path) { bundled_app("vendor/#{Bundler.ruby_scope}/gems") }
+
+    before do
+      FileUtils.mkdir_p(gems_path)
+      gemfile <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        gem 'rack'
+      G
+    end
+
+    it "should display a proper message to explain the problem" do
+      FileUtils.chmod("-x", gems_path)
+      bundle "config set --local path vendor"
+
+      begin
+        bundle :install, :raise_on_error => false
+      ensure
+        FileUtils.chmod("+x", gems_path)
+      end
+
+      expect(err).not_to include("ERROR REPORT TEMPLATE")
+
+      expect(err).to include(
+        "There was an error while trying to create `#{gems_path.join("rack-1.0.0")}`. " \
+        "It is likely that you need to grant executable permissions for all parent directories and write permissions for `#{gems_path}`."
+      )
+    end
+  end
+
   describe "when bundle cache path does not have write access", :permissions do
     let(:cache_path) { bundled_app("vendor/#{Bundler.ruby_scope}/cache") }