* Update makefile to use test/common/terminate_handler.cpp

This means we actually print error messages when using exceptions and
the makefile

* Better validate of GPU schedules

GPU loop constraints were checked in two different places. Checking them
in ScheduleFunctions was incorrect because it didn't consider update
definitions and specializations. Checking them in FuseGPUThreadLoops was
too late, because the Var names have gone (they've been renamed to
things like __thread_id_x). Furthermore, some problems were internal
errors or runtime errors when they should have been user errors. We
allowed 4d thread and block dimensions, but then hit an internal error.

This PR centralizes checking of GPU loop structure in
CanonicalizeGPUVars and adds more helpful error messages that print the
problematic loop structure. E.g:

```
Error: GPU thread loop over f$8.s0.v0 is inside three other GPU thread
loops. The maximum number of nested GPU thread loops is 3. The loop nest
is:
compute_at for g$8:
 for g$8.s0.v7:
  for g$8.s0.v6:
   for g$8.s0.v5:
    for g$8.s0.v4:
     gpu_block g$8.s0.v3:
      gpu_block g$8.s0.v2:
       gpu_thread g$8.s0.v1:
        gpu_thread g$8.s0.v0:
         store_at for f$8:
          compute_at for f$8:
           gpu_thread f$8.s0.v1:
            gpu_thread f$8.s0.v0:
```

Fixes the bug found in halide#7946

* Delete dead code

* Actually clear the ostringstream