Improve verbosity of panic messages in CsMat

[rth]

This improves verbosity of some panic messages in CsMat.

Currently, on the following example,

use sprs::CsMat;

let indptr = vec![0, 5, 8];
let indices = vec![761698, 328290, 828689, 761698, 780185, 901149, 780185, 144749, 258307];
let data = vec![1, 1, 1, 1, 1, 1, 1, 1, 1];
let a = CsMat::new((2, 1000000), indptr, indices, data);

I'm getting an obscure Indptr length does not match dimension panic message, so this improves the verbosity of such messages a bit, to show the lengths.

[rth]

rustfm is failing on the code I have not changed (and I did run cargo fmt prior to commiting).

[vbarrielle]

Thanks, that's more ergonomic indeed.