use PyErr_Format() instead of PyErr_SetString() for Cython generated exceptions with formatted but constant messages

error reporting: fix resource warning for unclosed file (in Py3.2+), cache source lines when we read them more than once

Conflicts:
	tests/errors/nogil.pyx

--HG--
extra : rebase_source : a88036d17a88ab216253c30e3f066a8697bc202e

Conflicts:
	Cython/Compiler/Main.py