Do not throw when luac returns an error code.

[vinsonchuong]

In luac 5.1, code errors cause an error code to be returned. So, "syntax error" error codes and "undefined global variable" error codes need to be allowed through. I wasn't able to find a list of luac error codes; so, I chose to remove the "if v:shell_error" guard completely.

This fixes Issue #3

[xolox]

Thanks for the feedback, I agree this should be improved. Unfortunately the fix is not quite as simple as your pull request makes it appear.

Note to myself and anyone reading along: I'd like to fix this but there's quite a bit of work involved. I'll have to adapt the interfaces of the following functions and check all call sites for each of these functions:

• The Vim script function xolox#misc#os#exec() defined in the vim-misc plug-in
• The Vim script function xolox#shell#execute() defined in the vim-shell plug-in
• The C functions execute_synchronous() and execute_asynchronous() in the DLL included in the vim-shell plug-in

I'll definitely implement this because it's been bothering me for quite a while that I can't reliably check return codes, but it might be a while until I have something that works well enough to publish.

[xolox]

This should now be fixed. Thanks for the feedback! If it still doesn't work for you, feel free to reopen this pull request or create a new issue.