Improved TemplateException error messages:

- TemplateException error messages are now much more readable when rendered in environments that don't obey to line-breaks. (This often happens in inproperly implemented HTML error pages and logs viewers.)
- TemplateException error messages are now contain up to 6 lines of FTL stack trace, because otherwise this information wasn't printed when the exception was a cause exception in a stack trace, of when only the value of getMessage() was printed instead of a stack trace.
- The meaning of the lines in the FTL stack trace is now self explanatory (lines start with "Failed at" and "Called by" instead of arrow and nothing).