Saving off entry string for use in panic

as the reader is not seekable and without this the panic string is
always empty :scream_cat:
Switching to io.MultiWriter for clarity
Adding tests for PanicLevel behavior, plus changing behavior!  :scream_cat:
Making panic level assertions more helpful
Merge pull request #76 from meatballhat/save-entry-string-for-panic

Panic with entry instead of entry string