Bugfix/2.8 formula up to constant

This allows error messages when doing the comparison of the adaptive function to the student answer so that errors like "Can't generate enough valid points" are passed on to the results table message column.  This resolves [bug 2776](http://bugs.webwork.maa.org/show_bug.cgi?id=2776).

Questions:
*  Do you want this to be a hotfix to master as well?
*  Since release/2.8 will eventually be merged back into develop, should we merge to develop now as well or wait for that?