Make rescuable asset error classes configurable

Consumers may be using a different asset pipeline than sprockets-rails,
which means that when `Importmap::Map#resolve_asset_paths` invokes the
application's `asset_path` helper method, it may need to rescue
different exceptions than `Sprockets::Rails::Helper::AssetNotFound`. To
support that, we've added a `rescuable_asset_errors` property to the
importmap configuration options. We add
`Sprockets::Rails::Helper::AssetNotFound` to the array if
`Sprockets::Rails` is defined, which should allow the gem to work the
same as before in applications that use sprockets-rails. But now it'll
also be usable in applications that use something other than
sprockets-rails.

Author: rmacklin

lib/importmap/engine.rb

@@ -7,6 +7,9 @@ module Importmap
   class Engine < ::Rails::Engine
     config.importmap = ActiveSupport::OrderedOptions.new
     config.importmap.sweep_cache = Rails.env.development? || Rails.env.test?
+    config.importmap.rescuable_asset_errors = [
+      defined?(Sprockets::Rails) && Sprockets::Rails::Helper::AssetNotFound,
+    ].compact
 
     config.autoload_once_paths = %W( #{root}/app/helpers )
 

lib/importmap/map.rb

@@ -88,7 +88,7 @@ def clear_cache
     end
 
     def rescuable_asset_error?(error)
-      error.is_a?(Sprockets::Rails::Helper::AssetNotFound)
+      Rails.application.config.importmap.rescuable_asset_errors.any? { |e| error.is_a?(e) }
     end
 
     def resolve_asset_paths(paths, resolver:)