Add #![feature(param_attrs)] to enable exception example

[bobgates]

On rust V1.39 nightly, at least, the exceptions example gives an error. Adding a line to the example allows it to run.

I'm very green at rust embedded, so there may be better ways, but this makes the example work.

[rust-highfive]

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[therealprof]

Hi @bobgates, I believe that is a fluke in the nightly build you're using because param_attrs where stabilized around that time, cf. rust-lang/rust#64010.

[bobgates]

Okay, with a current Rust nightly 1.44, this problem goes away.