Compiler warns about deprecated Error::description
Compiler warns about deprecated Error::description

This has been deprecated. It suggests either using `to_string` or the
`Display` impl which we have anyway.

mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-cfitsio,mindriot101/rust-fitsio,mindriot101/rust-fitsio,mindriot101/rust-cfitsio