Simplify check for byte encoding. Lowercase error message.

Hello Dominik,

I was just reading the code to see what it was doing and noticed this switch followed by an if, where both were doing the same thing.  So I thought I could change it to a single switch quickly and see what you think of it?

Also I've lowercased the error message, as I find it's more aesthetic when you pass errors up and golint also says that.