The standard says that formatted/unformatted input operations should
catch any exceptions, set iostate bits in the istream, and rethrow if
the exception mask says to do so.

We're failing to do that when the exception happens in the
istream::sentry constructor, while it extracts characters to skip
whitespace. This means that input operations exit with an exception
even when the mask says not to. The fix is simply to add a try/catch
to the sentry constructor.

I've also clarified some comments related to these semantics.

	PR libstdc++/53984
	* include/bits/basic_ios.h (basic_ios::_M_setstate): Adjust comment.
	* include/bits/istream.tcc (basic_istream::sentry): Handle exceptions
	during construction.
	* include/std/istream: Adjust comments for formatted input functions
	and unformatted input functions.
	* testsuite/27_io/basic_fstream/53984.cc: New.
	* testsuite/27_io/basic_istream/sentry/char/53984.cc: New.

Tested powerpc64le-linux, committed to trunk.