try to ensure all errors use spans

[graydon]

We have a bunch of cases of sess.err in the code that ought to be sess.span_err -- that is, they should carry a source position indicating the origin of the error. Fix these.

[lkuper]

I could only find one instance of sess.err that should have been sess.span_err, which I just committed a fix for. Otherwise, instances of ".err" are either invocations of the err method on a parser or reader object, which already use span_err, or they're errors along the lines of "multiple main fns defined" or "no input filename given", for which span doesn't make sense. See anything I missed?

[brson]

There are also plenty of places that just use fail directly and should be using span_err. I know extfmt does this.

[lkuper]

Should we have a policy that every fail should be replaced by a sess.span_err (or sess.unimpl or sess.bug)?

[brson]

lkuper: I think so