feature: Allow a default exception renderer to be specified (#837)

This could be used to preserve the old default of rendering
non-Liquid::Error messages or for providing default behaviour like error
reporting which could be missed if the exception renderer needed to be
specified on each render.

lib/liquid/context.rb

@@ -27,6 +27,7 @@ module Liquid
 
       @this_stack_used = false
 
+      self.exception_renderer = Template.default_exception_renderer
       if rethrow_errors
         self.exception_renderer = ->(e) { raise }
       end
@@ -78,9 +79,7 @@ module Liquid
       e.template_name ||= template_name
       e.line_number ||= line_number
       errors.push(e)
-
-      e = exception_renderer.call(e) if exception_renderer
-      e.to_s
+      exception_renderer.call(e).to_s
     end
 
     def invoke(method, *args)

lib/liquid/template.rb

@@ -69,6 +69,11 @@ module Liquid
       # :error raises an error when tainted output is used
       attr_writer :taint_mode
 
+      attr_accessor :default_exception_renderer
+      Template.default_exception_renderer = lambda do |exception|
+        exception
+      end
+
       def file_system
         @@file_system
       end

test/integration/error_handling_test.rb

@@ -211,6 +211,20 @@ class ErrorHandlingTest < Minitest::Test
     assert_equal [Liquid::InternalError], template.errors.map(&:class)
   end
 
+  def test_setting_default_exception_renderer
+    old_exception_renderer = Liquid::Template.default_exception_renderer
+    exceptions = []
+    Liquid::Template.default_exception_renderer = ->(e) { exceptions << e; '' }
+    template = Liquid::Template.parse('This is a runtime error: {{ errors.argument_error }}')
+
+    output = template.render({ 'errors' => ErrorDrop.new })
+
+    assert_equal 'This is a runtime error: ', output
+    assert_equal [Liquid::ArgumentError], template.errors.map(&:class)
+  ensure
+    Liquid::Template.default_exception_renderer = old_exception_renderer if old_exception_renderer
+  end
+
   def test_exception_renderer_exposing_non_liquid_error
     template = Liquid::Template.parse('This is a runtime error: {{ errors.runtime_error }}', line_numbers: true)
     exceptions = []