Consider adding error-codes

[jaybosamiya]

It would be useful for us to have each error reported by Verus come with an error code, similar to Rust (see https://doc.rust-lang.org/stable/error_codes/error-index.html).

This would allow users who face an error to quickly look-up more details, or easily jump to relevant part of code in Verus, and so on. Also, it would help create a point for documentation to build on, for example get more hints on ways to fix up the error and such.

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Opening this as a separate discussion from #387 since that one focuses on the specific error messages itself, but this is about updating all error codes.

[jaybosamiya]

My suggestion wrt the format itself would be to just use the format that Rust uses, i.e. E[0-9]{4} (eg: E0001, E0007, ...), but could start from a higher number, if we are worried about collisions with Rust's error codes.

Another alternative is to use V[0-9]{4} to indicate Verus.