Better Error handling

[ManoloTonto1]

I ran into an issue that my calendar did not have a getAllDayEndAt() property and I got an error which was property not found
I think it would be great to have a package with all the possible errors so we can better handle errors in production.

func (event *VEvent) getTimeProp(componentProperty ComponentProperty, expectAllDay bool) (time.Time, error) {
	timeProp := event.GetProperty(componentProperty)
	if timeProp == nil {
		return time.Time{}, errors.New("property not found")
	}

[arran4]

I think we can just add them to a file with consts, no need to introduce a whole new package. When my contract runs out I will consider this. Happy to review PRs and accept good ones too.

[arran4]

Once again: ManoloTonto1#2

It's getting a bit long

[arran4]

We are getting high risk of regression. So I will probably have to do something by this w/e with this