merge revision(s) r45178,r45180,r45183: [Backport #9568]

	eval.c: remove unneeded GC guard

	* eval.c (setup_exception): remove RB_GC_GUARD which is no longer
  needed since r41598.
	* eval.c (setup_exception): preserve errinfo across calling #to_s
	  method on the exception.  [ruby-core:61091] [Bug #9568]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@46236 b2dd03c8-39d4-4d8f-98ff-823fe69b080e