Improve error message of Liquid::MemoryLimit.

The existing error message is too generic since there are three types of limits in place.
It is useful to know which limit was reached to make it easier to debug the error.

lib/liquid/block_body.rb

@@ -192,8 +192,18 @@ module Liquid
 
     def raise_if_resource_limits_reached(context, length)
       context.resource_limits.render_length += length
-      return unless context.resource_limits.reached?
-      raise MemoryError, "Memory limits exceeded"
+
+      error_message =
+        if context.resource_limits.render_length_reached?
+          MemoryError::RENDER_LENGTH_ERROR_MESSAGE
+        elsif context.resource_limits.render_score_reached?
+          MemoryError::RENDER_SCORE_ERROR_MESSAGE
+        elsif context.resource_limits.assign_score_reached?
+          MemoryError::ASSIGN_SCORE_ERROR_MESSAGE
+        end
+
+      return unless error_message
+      raise MemoryError, error_message
     end
 
     def create_variable(token, parse_context)

lib/liquid/errors.rb

@@ -23,11 +23,14 @@ module Liquid
 
     def message_prefix
       str = +""
-      str << if is_a?(SyntaxError)
-        "Liquid syntax error"
-      else
-        "Liquid error"
-      end
+      str <<
+        if is_a?(SyntaxError)
+          "Liquid syntax error"
+        elsif is_a?(MemoryError)
+          "Liquid memory limit error"
+        else
+          "Liquid error"
+        end
 
       if line_number
         str << " ("
@@ -40,6 +43,12 @@ module Liquid
     end
   end
 
+  class MemoryError < Error
+    RENDER_LENGTH_ERROR_MESSAGE = 'Too many bytes rendered.'
+    RENDER_SCORE_ERROR_MESSAGE = 'Too many tags rendered.'
+    ASSIGN_SCORE_ERROR_MESSAGE = 'Too many bytes assigned to variables.'
+  end
+
   ArgumentError       = Class.new(Error)
   ContextError        = Class.new(Error)
   FileSystemError     = Class.new(Error)
@@ -47,7 +56,6 @@ module Liquid
   SyntaxError         = Class.new(Error)
   StackLevelError     = Class.new(Error)
   TaintedError        = Class.new(Error)
-  MemoryError         = Class.new(Error)
   ZeroDivisionError   = Class.new(Error)
   FloatDomainError    = Class.new(Error)
   UndefinedVariable   = Class.new(Error)

lib/liquid/resource_limits.rb

@@ -12,10 +12,16 @@ module Liquid
       reset
     end
 
-    def reached?
-      (@render_length_limit && @render_length > @render_length_limit) ||
-        (@render_score_limit && @render_score > @render_score_limit) ||
-        (@assign_score_limit && @assign_score > @assign_score_limit)
+    def render_length_reached?
+      @render_length_limit && @render_length > @render_length_limit
+    end
+
+    def render_score_reached?
+      @render_score_limit && @render_score > @render_score_limit
+    end
+
+    def assign_score_reached?
+      @assign_score_limit && @assign_score > @assign_score_limit
     end
 
     def reset