Fix unicode escape decode bug in `DoubleQuoteString` example.

[allenap]

Previously \u Unicode escape codes of between 4 and 6 characters were being rejected, whereas the error message indicates that the inverse should be the behavior.

The example has also been extended to use all recognized escape sequences (\n, \r, \t, \u) and give more information about what's happening.

[ok300]

Very useful, thanks!

@evancz please merge, this fixes an example and the output helps better understand what Parser did.