add Pry.last_internal_error

This is set to the last CommandError, SlopError, etc. Most often
raised by commands. This info is useful to pry devs debugging
otherwise opaque command errors.

lib/pry/pry_class.rb

@@ -55,6 +55,10 @@ class Pry
     # @return [Binding] A top level binding with no local variables
     attr_accessor :toplevel_binding
 
+    # @return [Exception, nil] The last pry internal error.
+    #   (a CommandError in most cases)
+    attr_accessor :last_internal_error
+
     # plugin forwardables
     def_delegators :@plugin_manager, :plugins, :load_plugins, :locate_plugins
 

lib/pry/pry_instance.rb

@@ -446,6 +446,7 @@ class Pry
   def process_command_safely(val)
     process_command(val)
   rescue CommandError, Slop::InvalidOptionError, MethodSource::SourceNotFoundError => e
+    Pry.last_internal_error = e
     output.puts "Error: #{e.message}"
     true
   end