Lab 3: Don't accept input with trailing garbage

Rerun the grammar that you created in lab 1. The input now
has trailing garbage:

     $19.95 Garbage

Notice that ANTLR builds the parse tree just fine (it
does report that it doesn't recognize "Garbage" but nonetheless
it proceeds to build the parse tree, without errors).

Then modify the grammar so that any garbage is illegal and
errors appear in the parse tree.