shill: wifi: Ignore NetworkUnknown error in supplicant RemoveNetwork.

BUG=chromium-os:35219
TEST=Build and run unit tests.

Change-Id: I7ba80d4549bd1df6beeb0c51f457bc8c6b3909e6
Reviewed-on: https://gerrit.chromium.org/gerrit/35532
Reviewed-by: Paul Stewart <pstew@chromium.org>
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/wpa_supplicant.cc b/wpa_supplicant.cc
index 06f197f..287fbf3 100644
--- a/wpa_supplicant.cc
+++ b/wpa_supplicant.cc
@@ -22,6 +22,7 @@
 const char kDBusPath[]              = "/fi/w1/wpa_supplicant1";
 const char kDriverNL80211[]         = "nl80211";
 const char kEnginePKCS11[]          = "pkcs11";
+const char kErrorNetworkUnknown[]   = "fi.w1.wpa_supplicant1.NetworkUnknown";
 const char kErrorInterfaceExists[]  = "fi.w1.wpa_supplicant1.InterfaceExists";
 const char kInterfacePropertyConfigFile[] = "ConfigFile";
 const char kInterfacePropertyCurrentBSS[] = "CurrentBSS";