Improve exception message for DontReadFromInput

[blueyed]

pytest/src/_pytest/capture.py

Lines 692 to 693 in 13a9d87

	def read(self, *args):
	raise IOError("reading from stdin while output is captured")

via #4996 (comment)

It should probably still contain the old string, and maybe even just emit another message before it, since this appears to be tested against at least with pytest itself.