Acceptance tests: Always check error after SshSession.exec()

The GitUtil.createProject() method in particular was failing silently
in a surprising number of cases. Fix all of these, and add the error
message to the AssertionError in cases where it was checked.

Change-Id: Ibca7881edfe8be583a19cc08f2b942bf4a9bbfc3

gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/GitUtil.java

@@ -100,6 +100,10 @@ public class GitUtil {
       b.append("\"");
     }
     s.exec(b.toString());
+    if (s.hasError()) {
+      throw new IllegalStateException(
+          "gerrit create-project returned error: " + s.getError());
+    }
   }
 
   public static Git cloneProject(String url) throws GitAPIException, IOException {