Error out on failed attribute type checks

Author: tarleb

src/resources/filters/modules/attribcheck.lua

@@ -141,9 +141,8 @@ local function warn_conversion (expected, actual, shift, extrainfo)
     dbginfostr = (dbginfo.name or '<no name>') .. ' in ' .. dbginfo.source
       .. ':' .. dbginfo.currentline
   end
-  warn(actual .. ' instead of ' .. expected .. ': ' .. dbginfostr
+  error(actual .. ' instead of ' .. expected .. ': ' .. dbginfostr
        .. (extrainfo and '\n' .. extrainfo or ''))
-  return
 end
 
 local ensure_type