add Pry::LastException.

pry adds behavior(methods) to the exception it returns at _pry_.last_exception
in a repl but when the exception has been passed along as a frozen object ruby
will raise an exception and pry fails to function as usual.

the commit changes that behavior so that the exception is wrapped in an instance of
Pry::LastException who is a subclass of BasicObject. Pry::LastException is a copy&paste
of what was defined dynamically in pry_instance.rb but it allows for frozen exceptions
to remain frozen and still provides the methods pry dynamically defined on an exception
object before. through a wrapper class pry also no longer modifies an exception object
that it doesn't own.

lib/pry.rb

@@ -184,3 +184,4 @@ require 'pry/terminal'
 require 'pry/editor'
 require 'pry/rubygem'
 require "pry/indent"
+require "pry/last_exception"

lib/pry/last_exception.rb

@@ -0,0 +1,35 @@
+class Pry::LastException < BasicObject
+  attr_reader :file, :line
+  attr_accessor :bt_index
+
+  def initialize(e)
+    @e = e
+    @bt_index = 0
+    @file, @line = bt_source_location_for(0)
+  end
+
+  def method_missing(name, *args, &block)
+    if @e.respond_to?(name)
+      @e.public_send(name, *args, &block)
+    else
+      super
+    end
+  end
+
+  def respond_to_missing?(name, boolean=false)
+    @e.respond_to?(name)
+  end
+
+  def pry?
+    true
+  end
+
+  def bt_source_location_for(index)
+    backtrace[index] =~ /(.*):(\d+)/
+    [$1, $2.to_i]
+  end
+
+  def inc_bt_index
+    @bt_index = (@bt_index + 1) % backtrace.size
+  end
+end

lib/pry/pry_instance.rb

@@ -467,27 +467,17 @@ class Pry
     self.last_result = result unless code =~ /\A\s*\z/
   end
 
+  #
   # Set the last exception for a session.
-  # @param [Exception] ex
-  def last_exception=(ex)
-    class << ex
-      attr_accessor :file, :line, :bt_index
-      def bt_source_location_for(index)
-        backtrace[index] =~ /(.*):(\d+)/
-        [$1, $2.to_i]
-      end
-
-      def inc_bt_index
-        @bt_index = (@bt_index + 1) % backtrace.size
-      end
-    end
-
-    ex.bt_index = 0
-    ex.file, ex.line = ex.bt_source_location_for(0)
-
+  #
+  # @param [Exception] e
+  #   the last exception.
+  #
+  def last_exception=(e)
+    last_exception = Pry::LastException.new(e)
     @last_result_is_exception = true
-    @output_array << ex
-    @last_exception = ex
+    @output_array << last_exception
+    @last_exception = last_exception
   end
 
   # Update Pry's internal state after evalling code.

spec/pry_spec.rb

@@ -67,6 +67,28 @@ describe Pry do
     end
   end
 
+  describe "#last_exception=" do
+    before do
+      @pry = Pry.new binding: binding
+      @e = mock_exception "foo.rb:1"
+    end
+
+    it "returns an instance of Pry::LastException" do
+      @pry.last_exception = @e
+      should.satisfy { @pry.last_exception.pry? == true }
+    end
+
+    it "returns a frozen exception" do
+      @pry.last_exception = @e.freeze
+      @pry.last_exception.should.be.frozen?
+    end
+
+    it "returns an object who mirrors itself as the wrapped exception" do
+      @pry.last_exception = @e.freeze
+      @pry.last_exception.should.be.instance_of?(StandardError)
+    end
+  end
+
   describe "open a Pry session on an object" do
     describe "rep" do
       before do