lua: Fix crash on unprotected lua errors (#12658)

Can be reproduced with a script like this:

-- in some lua file
vim.fn.timer_start(10, function() error("uh....") end)

-- will cause neovim to crash with the following error.
PANIC: unprotected error in call to Lua API
        (nlua_CFunction_func_call failed.)

After this, it will instead print the error message
from the top of the stack, like so.

tmp/error_nvim.lua:10: uh...

Also added an example test. Previously this test
caused the embedded nvim to panic.

src/nvim/lua/executor.c

@@ -987,7 +987,7 @@ int typval_exec_lua_callable(
   PUSH_ALL_TYPVALS(lstate, argvars, argcount, false);
 
   if (lua_pcall(lstate, argcount + offset, 1, 0)) {
-    luaL_error(lstate, "nlua_CFunction_func_call failed.");
+    nlua_print(lstate);
     return ERROR_OTHER;
   }