made test more tolerant of wording/punctuation changes

part of cleaning up larryl's fix for #54
fix signature_error doco due to signature_error_handler

part of cleaning up larryl's fix for #54
fixed typo I just happened to notice while grep'ing
refactoring sig_parsing_error for #63

I don't know if it will need to use signature_error_handler, but JIC it does, it should be easy now