Change error message for typo in helper's name

I've tried to make the error message more helpful, when helper name is misspelled. I created new type of error (MissingHelperModuleError) which is raised when expected constant is not loaded. I'd appreciate any feedback  about my solution.
Creating a custom error class seems excessive, when we can have a `NameError` with the message.
Also `safe_constantize` will return `nil` if constant doesn't exist. 

I'd rewrite this as:
```ruby
helper_name = file_name.camelize
helper_name.safe_constantize.tap do |const|
  raise NameError, "Couldn't find #{helper_name}, expected it to be " \
    "defined in app/helpers/#{file_name}.rb" unless const
end
```

Note this also rewords the error message slightly.

When this has been addressed :shipit: