Don't show the Debug representation of an error to the user

[cbourjau]

Its sad if things don't work out! At least we should make our error prettier 🌟 and not just print the Debug representation. Example of an error today (from #9 ):

Error: ErrorMessage { msg: "Unable to convert \'bench\' into a cargo subcommand" }

stack backtrace:
   0: failure::backtrace::internal::InternalBacktrace::new::h59b3db8423e4cf5e (0x10e71c9b8)
   1: <failure::backtrace::Backtrace as core::default::Default>::default::ha057354baab460c1 (0x10e71cc64)
   2: cargo_with::cargo::Cmd::from_strs::h41a41d93068aa265 (0x10e7123cd)
   3: cargo_with::runner::runner::h426b4b78cc407ce9 (0x10e717517)
   4: cargo_with::main::h57a9494064ed692b (0x10e70d932)
   5: std::rt::lang_start::{{closure}}::h6a72812a9432124a (0x10e71663a)
   6: std::panicking::try::do_call::hb3270f6ae846849d (0x10e80f837)
   7: ___rust_maybe_catch_panic (0x10e8215be)
   8: std::rt::lang_start_internal::ha07e743882e341f3 (0x10e8101ca)
   9: _main (0x10e70daf8)

[barskern]

I definitly agree on this one. Currently we use the return from main to print the error messages and it only uses the Debug implementation to print errors. "Fixing" this should be a minor change.