Be more robust when weird arguments are passed in

[matklad]

Two cases here:

• invalid utf8 -- this can happen in somewhat reasonable circumstances, so returning a graceful error is warranted here
• empty argv -- this one is weird, I think various things across the stack try to actually patch that up to never bubble empty argv to an application, but it's not to hard to add extra error handling here https://ryiron.wordpress.com/2013/12/16/argv-silliness/

Before:

01:22:39|~/p/argh|master✓
λ cargo run --example simple_example -- \Xff
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/examples/simple_example '�'`
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "\xFF"', library/std/src/env.rs:805:51
stack backtrace:
   0: rust_begin_unwind
             at /rustc/e080cc5a659fb760c0bc561b722a790dad35b5e1/library/std/src/panicking.rs:575:5
   1: core::panicking::panic_fmt
             at /rustc/e080cc5a659fb760c0bc561b722a790dad35b5e1/library/core/src/panicking.rs:65:14
   2: core::result::unwrap_failed
             at /rustc/e080cc5a659fb760c0bc561b722a790dad35b5e1/library/core/src/result.rs:1791:5
   3: core::result::Result<T,E>::unwrap
             at /rustc/e080cc5a659fb760c0bc561b722a790dad35b5e1/library/core/src/result.rs:1113:23
   4: <std::env::Args as core::iter::traits::iterator::Iterator>::next::{{closure}}
             at /rustc/e080cc5a659fb760c0bc561b722a790dad35b5e1/library/std/src/env.rs:805:35

After:

λ cargo run --example simple_example -- \Xff
    Finished dev [unoptimized + debuginfo] target(s) in 0.01s
     Running `target/debug/examples/simple_example '�'`
Invalid utf8: �

[matklad]

tests seem to be broken on master: #154