Challenges return error state

Partial backport of commit cdb23628.
Fixes issue #35.

acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java

@@ -19,6 +19,7 @@ import java.time.Instant;
 import java.util.Objects;
 
 import org.shredzone.acme4j.AcmeResource;
+import org.shredzone.acme4j.Problem;
 import org.shredzone.acme4j.Session;
 import org.shredzone.acme4j.Status;
 import org.shredzone.acme4j.connector.Connection;
@@ -47,6 +48,7 @@ public class Challenge extends AcmeResource {
     protected static final String KEY_STATUS = "status";
     protected static final String KEY_URI = "uri";
     protected static final String KEY_VALIDATED = "validated";
+    protected static final String KEY_ERROR = "error";
 
     private JSON data = JSON.empty();
 
@@ -117,6 +119,13 @@ public class Challenge extends AcmeResource {
         return data.get(KEY_VALIDATED).asInstant();
     }
 
+    /**
+     * Returns the reason why the challenge failed, if returned by the server.
+     */
+    public Problem getError() {
+        return data.get(KEY_ERROR).asProblem();
+    }
+
     /**
      * Returns the JSON representation of the challenge data.
      */

acme4j-client/src/test/java/org/shredzone/acme4j/challenge/ChallengeTest.java

@@ -113,6 +113,9 @@ public class ChallengeTest {
         assertThat(challenge.getStatus(), is(Status.VALID));
         assertThat(challenge.getLocation(), is(new URI("http://example.com/challenge/123")));
         assertThat(challenge.getValidated(), is(parseTimestamp("2015-12-12T17:19:36.336785823Z")));
+        assertThat(challenge.getError(), is(notNullValue()));
+        assertThat(challenge.getError().getType(), is(URI.create("urn:ietf:params:acme:error:connection")));
+        assertThat(challenge.getError().getDetail(), is("connection refused"));
         assertThat(challenge.getJSON().get("type").asString(), is("generic-01"));
         assertThat(challenge.getJSON().get("uri").asURL(), is(new URL("http://example.com/challenge/123")));
         assertThat(challenge.getJSON().get("not-present").asString(), is(nullValue()));

acme4j-client/src/test/resources/json.properties

@@ -225,7 +225,11 @@ genericChallenge = \
     "type":"generic-01", \
     "status":"valid", \
     "uri":"http://example.com/challenge/123", \
-    "validated":"2015-12-12T17:19:36.336785823Z" \
+    "validated":"2015-12-12T17:19:36.336785823Z", \
+    "error": { \
+      "type": "urn:ietf:params:acme:error:connection", \
+      "detail": "connection refused" \
+    }\
   }
 
 httpChallenge = \