Test invalid importmap file (#101)

* Raise custom error when we fail to parse importmap

Raising a custom error means it's a bit easier to track across error
monitoring tools.

* Rescue StandardError when parsing importmap files

We usually don't want to rescue the base Exception class as these errors
are reserved for things like a system interrupt. This commit swaps that out
for rescuing a StandardErrorr.

Author: AndyStabler

lib/importmap/map.rb

@@ -3,6 +3,8 @@
 class Importmap::Map
   attr_reader :packages, :directories
 
+  class InvalidFile < StandardError; end
+
   def initialize
     @packages, @directories = {}, {}
   end
@@ -11,9 +13,9 @@ def draw(path = nil, &block)
     if path && File.exist?(path)
       begin
         instance_eval(File.read(path), path.to_s)
-      rescue Exception => e
+      rescue StandardError => e
         Rails.logger.error "Unable to parse import map from #{path}: #{e.message}"
-        raise "Unable to parse import map from #{path}: #{e.message}"
+        raise InvalidFile, "Unable to parse import map from #{path}: #{e.message}"
       end
     elsif block_given?
       instance_eval(&block)

test/fixtures/files/invalid_import_map.rb

@@ -0,0 +1 @@
+typo "react", to: "https://ga.jspm.io/npm:[email protected]/index.js"

test/importmap_test.rb

@@ -56,6 +56,14 @@ def setup
     assert_match %r|assets/my_lib-.*\.js|, generate_importmap_json["imports"]["my_lib"]
   end
 
+  test 'invalid importmap file results in error' do
+    file = file_fixture('invalid_import_map.rb')
+    importmap = Importmap::Map.new
+    assert_raises Importmap::Map::InvalidFile do
+      importmap.draw(file)
+    end
+  end
+
   test "preloaded modules are included in preload tags" do
     preloading_module_paths = @importmap.preloaded_module_paths(resolver: ApplicationController.helpers).to_s
     assert_match /md5/, preloading_module_paths