Return an error instead of panicking when finalizing builders

[dbanty]

Specifically, I followed this example in order to create an Azure Blob SAS in tests, but didn't bother adding an expiration because I didn't realize it was required. Calling finalize() resulted in a panic due to this line. It would be a better experience to return a Result from finalize so it's clear that it's fallible and that some steps may be required.

Alternatively, it could fail during compilation either by using types to track which of the required steps have been completed or by putting the required parameters in the initial constructor function. That may be an even easier experience for consumers but I suspect is more work to add to the API.

[dbanty]

Just upgraded to the newer interfaces, thanks for fixing this!