Rationalize $status and errors

Prior to this fix, fish was rather inconsistent in when $status gets set
in response to an error. For example, a failed expansion like "$foo["
would not modify $status.

This makes the following inter-related changes:

1. String expansion now directly returns the value to set for $status on
error. The value is always used.

2. parser_t::eval() now directly returns the proc_status_t, which cleans
up a lot of call sites.

3. We expose a new function exec_subshell_for_expand() which ignores
$status but returns errors specifically related to subshell expansion.

4. We reify the notion of "expansion breaking" errors. These include
command-not-found, expand syntax errors, and others.

The upshot is we are more consistent about always setting $status on
errors.

src/fish_tests.cpp

@@ -1033,16 +1033,13 @@ static void test_1_cancellation(const wchar_t *src) {
         usleep(delay * 1E6);
         pthread_kill(thread, SIGINT);
     });
-    end_execution_reason_t ret = parser_t::principal_parser().eval(src, io_chain_t{filler});
+    eval_res_t res = parser_t::principal_parser().eval(src, io_chain_t{filler});
     auto buffer = io_bufferfill_t::finish(std::move(filler));
     if (buffer->buffer().size() != 0) {
         err(L"Expected 0 bytes in out_buff, but instead found %lu bytes, for command %ls\n",
             buffer->buffer().size(), src);
     }
-    // TODO: cancelling out of command substitutions is currently reported as an error, not a
-    // cancellation.
-    // do_test(ret == end_execution_reason_t::cancelled);
-    (void)ret;
+    do_test(res.status.signal_exited() && res.status.signal_code() == SIGINT);
     iothread_drain_all();
 }