Fixed undefined variable handling

Less substitutes variables in the toCSS phase of compilation, not the parsing stage, and if a variable doesn't exists, it throws an exception.  This exception was never handled.

The result of this was that the error was reported as an ordinary exception, with enough detail when node was being used that it didn't really matter (it just wasn't reported nicely), but when trireme was being used, no detail was shown.

This fixes the problem by adding a try/catch around the conversion to CSS, and handling errors from that in the same way as parsing errors are handled.

Additionally, I did the following:

* Updated all scripted tests to use SBT 0.13.5 instead of 0.13.5-RC3
* Refactored error output test to run comprehensive assertions on error output, including line number, error message and file contents
* Made error output test run on both node and trireme, but only if node is installed
* Fixed an off by one error in loading the contents of files at the reported line number