raze panics when Cargo.toml doesn't contain raze metadata

[gkelly]

Pretty easy to replicate. cargo new somewhere and then cargo raze it:

gkelly@praxis> cargo new --bin razor
     Created binary (application) `razor` package
gkelly@praxis> cd razor
gkelly@praxis> cargo raze
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', src/settings.rs:640:24
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[gkelly]

With #378 merged this now looks like:

gkelly@praxis> cargo raze
Error: Raze failed with cause: "No raze settings were specified in the Cargo.toml file, see README.md for details on expected fields"