Changed SafeProbScores to return an error instead of a panic

I know this is not a backwards compatible change, but this is a mathematical error and not really a runtime error, so:
a) a panic causes functions to unwind outside of this package, which is not good for long running applications and 
b) there's little need to fill the log with stack traces given this is a known and reasonably common outcome.

PS. Great package, really useful, thanks!