flags: better error message on invalid reload url

[kevinkassimo]

Noticed this during slides prep.

Before:

$ deno -r=123 https://deno.land/std/examples/welcome.ts
thread 'main' panicked at 'Bad Url: 123', cli/flags.rs:1238:7
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After:

./target/debug/deno -r=123 https://deno.land/std/examples/welcome.ts
Unable to resolve URL '123': relative URL without a base

[bartlomieju]

This method should rather return Result<Vec<String>, Error>; otherwise there's no way to write unit test for failing URL - process will exit instead of erroring and running other tests

[ry]

I agree. We shouldn't make it more ergonomic to process::exit(1), instead we should be removing those.

@kevinkassimo I appreciate the patch as always - but I'd rather the ugly panic than introducing this exit-macro. Closing without merge.