Remove uses of std::process::exit

[jfecher]

Problem

std::process::exit prevents drops from running which has the potential to be problematic for e.g. open files or other IO relying on being closed cleanly.

Solution

Remove each use of std::process::exit, instead percolating the error up to nargo's main function and returning an error there.

Additional context

The nargo test PR brings us a bit closer in percolating up errors more to be caught by testing, but also adds more std::process::exit instances in doing so.

[phated]

This is also important for things that get compiled to wasm, as std::process::exit causes a force exit in all WASI runtimes and breaks a lot of tooling (I had to deal with this in Circom).

[kevaundray]

Assigning this to @Ethan-000