Add error handling to examples

[craigpastro]

The error after calling jwt.Parse is often omitted while other errors are checked. The lack of consistency can be a bit confusing I think, so this PR adds some of the checks in.

Resolves #309 I suppose.

[oxisto]

Thanks! It's a start, but I am still not really too happy about the whole situation where we have an err and token.Valid. They pretty much represent the same information, i.e., if there was no error, token.Valid will always be true. However, if there is an error, token.Valid is most likely. be true, but it could also be that the error happens before token validation, and thus token is nil.

I would even lean towards only checking for err and leaving out checks to token.Valid

[craigpastro]

👋 @oxisto. Thanks for the review! That makes good sense. I removed all the token.Valid checks after checking the error.

[oxisto]

I adjusted the use of else-if consistently, looks good then