feat: push_str capacity exceeds error message

[hlsxx]

First off, great work on the library! The source code is very clean and nicely structured :)

I improved ExceedsCapacity error message clarity. I made a small change to improve the ExceedsCapacity error message, which was a bit confusing in its current form.

In current state:

let mut x = qstr::BStr7::new();
x.push_str("Hello world").unwrap();

Error message: String length (0) exceeds capacity (7)

This may be misleading because it shows the current string length (0) instead of the total length that the operation should result in (11). (In my opinion)

After this change
Error message: String length (11) exceeds capacity (7)

The error now displays the total resulting length (current length + new string length) instead of just the initial value.

[tindzk]

Thanks for the contribution!

Could you please add a test for this change in src/tests/bounded_str_tests.rs?

[tindzk]

Thanks! I released v0.1.2 with this fix.