Don't swallow exceptions when downloading stubs

without this change when any exception occurs (e.g. the stub can't be downloaded) a null will be returned
with this change we want to throw an exception instead

fixes #314

spring-cloud-contract-stub-runner/src/main/java/org/springframework/cloud/contract/stubrunner/AetherStubDownloader.java

@@ -165,13 +165,11 @@ public class AetherStubDownloader implements StubDownloader {
 			throw ise;
 		}
 		catch (Exception e) {
-			//TODO: Start throwing this exception instead of returning null
-			log.warn(
+			throw new IllegalStateException(
 					"Exception occurred while trying to download a stub for group ["
 							+ stubsGroup + "] module [" + stubsModule
 							+ "] and classifier [" + classifier + "] in " + this.remoteRepos,
 					e);
-			return null;
 		}
 
 	}

spring-cloud-contract-stub-runner/src/test/groovy/org/springframework/cloud/contract/stubrunner/AetherStubDownloaderSpec.groovy

@@ -11,7 +11,7 @@ import spock.util.environment.RestoreSystemProperties
 class AetherStubDownloaderSpec extends Specification {
 
     @Rule
-    HoverflyRule hoverflyRule = HoverflyRule.inCaptureMode("simulation.json")
+    HoverflyRule hoverflyRule = HoverflyRule.inSimulationMode("simulation.json")
 
     def 'Should be able to download from a repository using username and password authentication'() {
         given:
@@ -31,6 +31,22 @@ class AetherStubDownloaderSpec extends Specification {
         jar != null
     }
 
+    def 'Should throw an exception when artifact not found'() {
+        given:
+        StubRunnerOptions stubRunnerOptions = new StubRunnerOptionsBuilder()
+            .withWorkOffline(true)
+            .build()
+
+        AetherStubDownloader aetherStubDownloader = new AetherStubDownloader(stubRunnerOptions)
+
+        when:
+        def jar = aetherStubDownloader.downloadAndUnpackStubJar(new StubConfiguration("non.existing.group", "missing-artifact-id", "1.0-SNAPSHOT"))
+
+        then:
+        IllegalStateException e = thrown(IllegalStateException)
+        e.message.contains("Exception occurred while trying to download a stub for group")
+    }
+
     def 'Should throw an exception when a jar is in local m2 and not in remote repo'() {
         given:
         StubRunnerOptions stubRunnerOptions = new StubRunnerOptionsBuilder()