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.

tests/checks/eval.fish

@@ -21,14 +21,14 @@ echo $status
 false
 eval "("
 echo $status
-# CHECK: 1
+# CHECK: 123
 # CHECKERR: {{.*}}checks/eval.fish (line {{\d+}}): Unexpected end of string, expecting ')'
 # CHECKERR: (
 # CHECKERR: ^
 false
 eval '""'
 echo $status
-# CHECK: 1
+# CHECK: 123
 # CHECKERR: {{.*}}checks/eval.fish (line {{\d+}}): The expanded command was empty.
 # CHECKERR: ""
 # CHECKERR: ^