Auto merge of #9600 - danlrobertson:opts_parser_err, r=ecoal95

Add informative error messages when parsing command line options returns an error

The background behind why I'm submitting this PR is slightly embarrassing. After running `./mach` with some servo options I got the following stack backtrace.

```
thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: ParseIntError { kind: InvalidDigit }', ../src/libcore/result.rs:746
stack backtrace:
   1:     0x56459001b0b0 - sys::backtrace::tracing::imp::write::hb120982054a416e35nu
   2:     0x56459001e02b - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.42840
   3:     0x56459001dc96 - panicking::default_handler::h7ae2e4523ef4c187IFy
   4:     0x56459000760c - sys_common::unwind::begin_unwind_inner::h5fb19481d14902dbDgt
   5:     0x564590007cf8 - sys_common::unwind::begin_unwind_fmt::hd62d57279546b0f2Jft
   6:     0x56459001a701 - rust_begin_unwind
   7:     0x56459004c01f - panicking::panic_fmt::h27f7225e08792f40qYL
   8:     0x56458f393901 - result::unwrap_failed::h18215968003310890981
                        at ../src/libcore/macros.rs:29
   9:     0x56458f3937b9 - result::Result<T, E>::unwrap::h10319050269194353824
                        at ../src/libcore/result.rs:687
  10:     0x56458f383701 - opts::from_cmdline_args::h8ea8d8c87dc6ee726lg
                        at /home/drobertson/git/servo/components/util/opts.rs:599
  11:     0x56458d1f8cdf - main::h8517eb49d15e90fbNaa
                        at /home/drobertson/git/servo/components/servo/main.rs:53
  12:     0x56459001d8f4 - sys_common::unwind::try::try_fn::h9076850075504893162
  13:     0x56459001a68b - __rust_try
  14:     0x56459001d36e - rt::lang_start::h58a22f304b0c1e19Oxy
  15:     0x56458d2f4aa9 - main
  16:     0x7f370496360f - __libc_start_main
  17:     0x56458d1c04a8 - _start
  18:                0x0 - <unknown>
Servo exited with return value 101
```

I immediately opened up `gdb` and got to work only to realize that the error was due to a typo in an option that had been `unwrap`ed at [components/util/opts.rs:599](https://github.com/servo/servo/blob/master/components/util/opts.rs#L599). Perhaps some more informative error messages will help prevent some future face-palm moments like mine :smile:

I couldn't think of a good way to add tests for this. Please let me know if I missed something. Comments and critiques are welcome!

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