libstdc++: Add pretty printer for std::error_code and std::error_condition

Signed-off-by: Jonathan Wakely <jwakely@redhat.com>

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdErrorCodePrinter): Define.
	(build_libstdcxx_dictionary): Register printer for
	std::error_code and std::error_condition.
	* testsuite/libstdc++-prettyprinters/cxx11.cc: Test it.