Issues compiling with exceptions disabled

[andy-byers]

I've noticed the following two things when compiling with -fno-exceptions:

1. An unused parameter warning on line 202 in expected.hpp (using b74fecd for reference, but it's the same with v1.0.0)
2. The second one is a "use of a deleted function" error (I added a test in my PR that should demonstrate it)

Hopefully that makes sense. Thanks for the great library!

[andy-byers]

Everything works as expected when compiling with -fno-exceptions