Improve diagnostics for failed static_assert!

[ojeda]

From #269 (comment):

One small nit though: when the static assertion fails, the build error is very cryptic. It's not immediately obvious what has happened.

const MY_ERRNO: u32 = 40000;
static_assert!((-(MY_ERRNO as i128)) fits in i16);

error: any use of this value will cause an error
   --> rust/kernel/static_assert.rs:129:23
    |
129 |         const _: () = [()][!($condition) as usize];
    |         --------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
    |                       |
    |                       index out of bounds: the length is 1 but the index is 1
    | 
   ::: rust/kernel/error.rs:113:1
    |
113 | static_assert!((-(MY_ERRNO as i128)) fits in i16);
    | -------------------------------------------------- in this macro invocation
    |
    = note: `#[deny(const_err)]` on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #71800 <https://github.com/rust-lang/rust/issues/71800>
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

[alex]

This should be done by #778