panic_implementation: Error message talks about the panic_impl lang item when missing

[phil-opp]

If the new panic_implementation attribute is missing, the error message mentions the a panic_impl lang item:

error: language item required, but not found: `panic_impl`

Instead it should probably mention that a panic_implementation function is missing.

Edit: Seems like I didn't read it carefully enough. The error message doesn't mention the old panic_fmt language item, but a new panic_impl language item. Either way, a error message mentioning the panic_implementation attribute would be better.

[japaric]

Under the hood we have replaced the old panic_fmt with a new panic_impl lang item but this is an implementation detail of #[panic_implementation]. I think we can special case the error message for missing panic_impl to, instead, mention #[panic_implementation] but, off the top of my head, I don't know where that would be in the code base.