NullPointer in HttpClient for IOException but no errorStream

[bweefting]

We get an error-logging from within Prismic.
As this happens while handling an exception before logging it, it hides the actual issue.

`
package io.prismic.core;
public class HttpClient {

public static JsonNode fetch(String url, Logger logger, Cache cache, Proxy proxy) {
....
} catch (IOException e) {
String body;
String errorText = "Unknown error";
JsonNode errorJson = new ObjectMapper().readTree(httpConnection.getErrorStream());
`

The httpConnection.getErrorStream is (per documentation) possibly NULL.
The readTree() method however does not support that.

Seems at least a simple-NULL-check could prevent this.

Another PR (#69) is open for quite some time.
Is there any activity on this repo, if alternative libraries are available would like to learn that :)