Make liquid error rendering optional.

Although the author of the liquid template wants to see these errors, they
probably don't want the visitor to see the liquid errors.  Probably the
best fallback when rendering the page for visitors is to render the empty
string for tags with errors.

lib/liquid/context.rb

@@ -13,7 +13,7 @@ module Liquid
   #   context['bob']  #=> nil  class Context
   class Context
     attr_reader :scopes, :errors, :registers, :environments, :resource_limits
-    attr_accessor :exception_handler
+    attr_accessor :exception_handler, :render_errors
 
     def initialize(environments = {}, outer_scope = {}, registers = {}, rethrow_errors = false, resource_limits = nil)
       @environments     = [environments].flatten
@@ -22,6 +22,7 @@ module Liquid
       @errors           = []
       @resource_limits  = resource_limits || ResourceLimits.new(Template.default_resource_limits)
       squash_instance_assigns_with_environments
+      @render_errors    = true
 
       @this_stack_used = false
 
@@ -69,7 +70,7 @@ module Liquid
 
       errors.push(e)
       raise if exception_handler && exception_handler.call(e)
-      Liquid::Error.render(e)
+      render_errors ? Liquid::Error.render(e) : ''
     end
 
     def invoke(method, *args)