Don't add expansion error offset twice

Like the $status commit, this would add the offset to already existing
errors, so

```fish
(foo)
(bar)

something
```

would see the "(foo)" error, store the correct error location, then
see the "(bar)" error, and *add the offset of (bar)* to the "(foo)"
error location.

Solve this by making a new error list and appending it to the existing
ones.

There's a few other ways to solve this, including:

- Stopping after the first error (we only display the first anyway, I
think?)
- Making it so the source location has an "absolute" flag that shows
the offset has already been added (but do we ever need to add two offsets?)

I went with the simpler fix.

tests/checks/syntax-error-location.fish

@@ -10,3 +10,15 @@ $status
 # CHECK: <fish: The 'exec' command can not be used in a pipeline>
 # CHECK: <echo foo | exec grep # this exec is not allowed!>
 # CHECK: <           ^>
+
+echo '
+
+(true one)
+(true two)
+
+# more things
+' | $fish 2>| string replace -r '(.*)' '<$1>'
+
+# CHECK: <fish: Command substitutions not allowed>
+# CHECK: <(true one)>
+# CHECK: <^>