Move the p2p error types to the bottom of the file next to the various
impls for these types.

Code move only, no other changes.

By convention we always include the suffix "Error" on our error types.

Rename the error type `UnknownChainHash` to `UnknownChainHashError`.

By convention we always include the suffix "Error" on our error types.

Rename the error type `UnknownMagic` to `UnknownMagicError`.

By convention we always include the suffix "Error" on our error types.

Rename the error type `HiddenNodes` to `HiddenNodesError`.

We would like the codebase to be optimized for readability not ease of
development, as such code that is write-once-read-many should not use
macros.

Currently we use the `impl_std_error` macro to implement
`std::error::Error` for struct error types. This makes the code harder
to read at a glance because one has to think what the macro does.

Remove the `impl_std_error` macro and write the code explicitly.

In a further effort to make the code brain-dead easy to read; use an
explicit implementation of `std::error::Error` that returns `None`
instead of relying on the default trait implementation.

On our way to v1.0.0 we are defining a standard for our error types,
this includes:

- Uses the following derives (unless not possible, usually because of `io::Error`)

  `#[derive(Debug, Clone, PartialEq, Eq)]`

- Has `non_exhaustive` unless we really know we can commit to not adding
  anything.

Furthermore, we are trying to make the codebase easy to read. Error code
is write-once-read-many (well it should be) so if we make all the error
code super uniform the users can flick to an error and quickly see what
it includes. In an effort to achieve this I have made up a style and
over recent times have change much of the error code to that new style,
this PR audits _all_ error types in the code base and enforces the
style, specifically:

- Is layed out: definition, [impl block], Display impl, error::Error impl, From impls
- `error::Error` impl matches on enum even if it returns `None` for all variants
- Display/Error impls import enum variants locally
- match uses *self and `ref e`
- error::Error variants that return `Some` come first, `None` after

Re: non_exhaustive

To make dev and review easier I have added `non_exhaustive` to _every_
error type. We can then remove it error by error as we see fit. This is
because it takes a bit of thinking to do and review where as this patch
should not take much brain power to review.