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:

Closes #2241 (closed)

@DouweM Let me know if you have better ideas on how to format the error messages (and perhaps we should keep the height of the row the same).

Status changed to merged

mentioned in commit 2f706fbd

I'm on mobile so I was a bit too quick to merge :/ for the error messages, using errors.full_messages may be better. We also don't need the red background, I think using a fontawesome error icon with text is fine.

mentioned in merge request !1138 (merged)

Also, I still don't quite understand why Rails was resolving namespace_projects_path in the error message, rather than using the correct namespace_project_path. Maybe it has something to do with this bug? I'm not sure how import ever worked before.

mentioned in commit f4cb6e43