Replace err_derive dependency with manual impl of Display + Error

[faern]

Building this library on beta or nightly yields the following warning:

warning: non-local `impl` definition, `impl` blocks should be written at the same level as their item
  --> nftnl/src/batch.rs:8:10
   |
8  | #[derive(err_derive::Error, Debug)]
   |          ^----------------
   |          |
   |          `Error` is not local
   |          move the `impl` block outside of this constant `_DERIVE_std_error_Error_FOR_NetlinkError`
9  | #[error(display = "Error while communicating with netlink")]
10 | pub struct NetlinkError(());
   |            ------------ `NetlinkError` is not local

We only have a single error in this crate, and it's an extremely simple one. Here I just replace this derive macro with a simple manual implementation! Less weird bugs and much more future proof! And fewer dependencies! 🥳 This change removes eleven dependencies from the dependency tree of this (should be) small library.

Getting rid of these warnings is a prerequisite for getting #57 into a working state.

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This change is 

[Serock3]

🙌

Reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved