API: return non-generic VimL errors

- Return VimL errors instead of generic errors for:
  - nvim_call_function
  - nvim_call_dict_function
- Fix tests which were silently broken before this change.

This violates #6150 where we agreed not to translate API errors.  But
that can be fixed later.

test/functional/api/server_requests_spec.lua

@@ -250,7 +250,7 @@ describe('server -> client', function()
     end)
 
     after_each(function()
-      funcs.jobstop(jobid)
+      pcall(funcs.jobstop, jobid)
     end)
 
     if helpers.pending_win32(pending) then return end