strip backtrace on parsing error

Before this patch we throw an error with backtrace:
```
/opt/cartridge/.rocks/share/tarantool/graphql/parse.lua:6: 4.3: syntax error, unexpected }
```

Actually seems we don't need it. Let's stip it.

Author: olegrok

graphql/parse.lua

@@ -3,7 +3,7 @@ local luagraphqlparser = require('luagraphqlparser')
 local function parse(s)
   local ast, err = luagraphqlparser.parse(s)
   if err ~= nil then
-    error(err)
+    error(err, 0)
   end
   return ast
 end