repl: Fix error() calls with a non-string message breaking the repl

[squeek502]

Fixes #898

error() calls with non-string messages will now output the following:

Welcome to the Luvit repl!
> error(function() end)
error with unexpected error message type (function), no stacktrace available
> error()
error with unexpected error message type (nil), no stacktrace available

Let me know if you think there's a better way to handle this, or if the message could be made clearer.

[creationix]

Maybe instead of the type, run the value through tostring?

[squeek502]

Using tostring would be largely the same except in the case of boolean, where the output would be something like error with unexpected error message type (false), no stacktrace available, which seems like it could be confusing.

I'm not sure there's much benefit to displaying the value, but I'll leave it up to you.

For reference, here's what the plain Lua 5.1.5 repl outputs (note that error() is totally silent):

Lua 5.1.5  Copyright (C) 1994-2012 Lua.org, PUC-Rio
> error()
> error(false)
(error object is not a string)
> error(function() end)
(error object is not a string)