Add better errors gem

Add better errors for consistency with other apps
Merge pull request #37 from alphagov/tech/add-better-errors

Add better errors gem
Require better_errors only in development

The better_errors gem is not available in other environments so only require it in development mode.
Add some default properties to schema mappings

Most importantly is the `updated_at` field, which is used to sort the
results if no keyword term is provided. Because this field doesn't exist
until the first entry has been added, the elasticsearch query causes an
error.
Merge pull request #39 from alphagov/add-default-properties-to-schemas

Add some default properties to schema mappings
Merge pull request #38 from alphagov/tech/add-better-errors

Require better_errors only in development
Remove better_errors config require from rake task

The rake task doesn't load Sinatra so it doesn't have `configure`. Plus,
since this is a rake task to be run from the console, having an HTML
error page isn't actually very useful.
Merge pull request #40 from alphagov/remove-better-errors-config-from-rake-task

Remove better_errors config require from rake task