What was the intended difference between int_error(), os_error(), and graph_error()?

[sfeam <sf...@us...>]

This question comes from discussion of Bug #1741.

There are about 675 instances of calling int_error() in the gnuplot source code.
There are about 25 instances each of calls to os_error() or graph_error().
All three error paths do roughly the same thing except that graph_error() terminates
multiplot mode before proceeding.

Does anyone recall the original intent of having 3 different error paths?

Most of the os_error() call sites have to do with file handling, but there are many
other file errors that go through int_error().  If that was the intended distinction
it has been lost over several decades of development.

Bug #1741 points out that of the 3 error paths, only int_error() sets
GPVAL_ERRNO.  That obviously could be fixed, but the larger question
is whether there is any reason to keep the two little-used error paths at all?

	Ethan