Errors ignored in DecodePolyline

[rosshemsley]

Looks like errors are printed to stderr instead of returned in the polyline decoder:

https://github.com/googlemaps/google-maps-services-go/blob/master/polyline.go#L49

This means it's not possible to tell if the whole polyline was successfully decoded.

A nicer API would be to propagate the errors up (and leave logging to stderr to the caller)

edit: actually this is log.Fatal, so if you try and parse an invalid polyline it will call os.Exit(1) - which seems non-ideal..

[domesticmouse]

Merged a fix that exposes failing polyline decodes as an error.