Lexer: Provide more helpful error message for trailing colon

If you accidentally add a colon at the end of a line, like in Python, the CoffeeScript compiler (HEAD) produces rather unhelpful error messages. I ended up spending some time yesterday hunting down an extra colon in the middle of my source. ;-)

```
$ bin/coffee -sc
while 1:
1
Error: Parse error on line 2: Unexpected 'TERMINATOR'

$ bin/coffee -sc
for i in [1]:
1
Error: Parse error on line 1: Unexpected '{'
```

This is not really a bug, more like a nuisance -- just thought you might appreciate the report. Feel free to close it if there is nothing that can be done.

Reproduce the two examples with bin/coffee -sc and inspect the lexer/parser error-handling path for trailing colons. Done means both examples report a helpful, specific diagnostic instead of the current unexpected-token errors, without regressing normal CoffeeScript syntax.

Written by the indexing model from the issue text.