Fix broken code import and display error messages if something went wrong with creating project

Clicking "Import" in Bitbucket import status page would result in:

```
  Rendered import/base/create.js.haml (38.3ms)
Completed 500 Internal Server Error in 1362ms (ActiveRecord: 20.2ms)

NoMethodError - undefined method `namespace_projects_path' for #<#<Class:0x007fd6c0f29498>:0x007fd6c0cf09d8>:
  actionpack (4.1.11) lib/action_dispatch/routing/polymorphic_routes.rb:142:in `polymorphic_url'
  actionpack (4.1.11) lib/action_dispatch/routing/polymorphic_routes.rb:148:in `polymorphic_path'
  actionview (4.1.11) lib/action_view/routing_url_for.rb:87:in `url_for'
  turbolinks (2.5.3) lib/turbolinks/xhr_url_for.rb:12:in `url_for_with_xhr_referer'
  actionview (4.1.11) lib/action_view/helpers/url_helper.rb:181:in `link_to'
  app/views/import/base/create.js.haml:23:in `_app_views_import_base_create_js_haml__1092746368522631377_70280180507700'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:161:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:143:in `render'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionview (4.1.11) lib/action_view/renderer/template_renderer.rb:55:in `block (2 levels) in render_template'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
```

Also, if the project failed to save for some reason, the user was never informed. Screenshot of new error message dump:

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/afc15aa6b0cde0080976eab3199cf18b/image.png)

Closes #2241

See merge request !1134