shill: implement Manager.GetService (error-case only)

BUG=chromium-os:20254
TEST=unittests, WiFiManager/000_SSID_Length_Limit

this gives us enough to pass the autotest for
network_WiFiManager/000_SSID_Length_Limit.

Change-Id: Ib0305e707d2203327d846be3e0b206033d6a884a
Reviewed-on: http://gerrit.chromium.org/gerrit/7567
Commit-Ready: mukesh agrawal <quiche@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/error.cc b/error.cc
index 030e598..e486bb5 100644
--- a/error.cc
+++ b/error.cc
@@ -65,9 +65,12 @@
   message_ = message;
 }
 
-void Error::ToDBusError(::DBus::Error *error) const {
+bool Error::ToDBusError(::DBus::Error *error) const {
   if (IsFailure()) {
     error->set(GetName(type_).c_str(), message_.c_str());
+    return true;
+  } else {
+    return false;
   }
 }