fix: input validation to NewRequest

[athyk]

Fixes panic: runtime error: invalid memory address or nil pointer dereference when passing http.Request that has not defined r.Host or r.URL.Path or even just nil in general.

req := &http.Request{}
sentry.NewRequest(req)

• This adds an error signature onto NewRequest and entryToEvent
• Tests have been added to cover error handling

[ribice]

This is a breaking change, as it changes the signature of NewRequest. I assume that's used externally (as it's public/exposed). The added validation doesn't seem worthy of a breaking change.