Ensure type-level Javadoc in every class, comply with guidelines for
80 char on Javadoc, and minor polish.

The `WebClient` has a new exception hierarchy:

* `WebClientException` is the root of all exceptions thrown by the
`WebClient`
* `WebClientResponseException` are all exceptions associated with
specific HTTP response status codes
* `WebClientErrorException` and `WebServerErrorException` are
respectively for 4xx and 5xx HTTP status codes

`ResponseExtractor` implementations are adapted to optionally throw
exceptions if it's impossible to extract the relevant parts of the
response (e.g. extracting the response body if the response is a 404).

This commit also introduces `ResponseErrorHandler`s that take care of
the whole exception mapping infrastructure. Since
`WebClientResponseException`s provide the status, headers and response
body, we also need a dedicated mechanism to extract information from the
response body at that level.

The `BodyExtractors` are responsible for extracting that information
from the exception, given they are provided with all the information
they need; in that case, message decoders are required.

To convey all this new information downstream, the `WebClient` now wraps
the message converters and response error handler instances into a
dedicated `WebClientConfig` object.