Allow for more descriptive errors during parsing

[jpalawaga]

I had a need to improve the error handling around the Parse function. Specifically, if Itoa failed it yield a nasty error.

I've only overhauled the cases that I need, but I'm happy to invent a couple other error types for completeness for the rest of parse.go, but only if this changelist is something along the lines that you'd actually care to accept. If it isn't something you're interested in, no worries.

Thanks for building the package!