Expose *really good* error messages

[davepeck]

Right now the error messages we expose are... not really good:

1. Slight changes in t-string content can lead to radically different errors (one might hit the HTML parser; the other might hit the t-string processor; etc)
2. None of the errors shows exactly where in your t-string things went wrong
3. We can definitely write better text for all the error cases

This is a big effort, particularly if we want to show exactly where things are going wrong.

[davepeck]

With the merge of #83 we have the pieces in place (in the SourceTracker) to do better here. I consider this my next task, I think.