account for the pick-constraint edges when reporting errors

Also, thread through better span info to improve the error message to
something tolerable.

src/test/ui/impl-trait/multiple-lifetimes/error-handling.rs

@@ -0,0 +1,21 @@
+// compile-flags:-Zborrowck=mir
+
+#![feature(existential_type)]
+
+#[derive(Clone)]
+struct CopyIfEq<T, U>(T, U);
+
+impl<T: Copy> Copy for CopyIfEq<T, T> {}
+
+existential type E<'a, 'b>: Sized;
+//~^ ERROR lifetime may not live long enough
+
+fn foo<'a, 'b, 'c>(x: &'static i32, mut y: &'a i32) -> E<'b, 'c> {
+    let v = CopyIfEq::<*mut _, *mut _>(&mut {x}, &mut y);
+    let u = v;
+    let _: *mut &'a i32 = u.1;
+    unsafe { let _: &'b i32 = *u.0; }
+    u.0
+}
+
+fn main() {}