libexpr: add set-to-string error frame

this is much more useful than a frame pointing to the set but claiming
it as a function being called. if the function is actually at fault we
will now point to its attribute, although the position may be slightly
wrong if __toString was defined from set updates or builtin functions.

Change-Id: Ib3eb237a276d94426d9c6fc0e26eea72382d34a2

tests/functional/lang/eval-fail-toString.nix

@@ -0,0 +1 @@
+toString { __toString = 1; }