shill: move passphrase validation from WiFi to WiFiService

this paves the way for actually parsing passphrase info into
fields of a WiFiService, rather than just validating it. also,
we need this code for calls to WiFiService.SetProperty.

BUG=None
TEST=unittests

Change-Id: I8467b36e07588b99f12cc5a4ecb7cf2deb7cf068
Reviewed-on: http://gerrit.chromium.org/gerrit/8821
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
diff --git a/key_value_store.cc b/key_value_store.cc
index bc0f55b..d905467 100644
--- a/key_value_store.cc
+++ b/key_value_store.cc
@@ -24,13 +24,13 @@
 
 bool KeyValueStore::GetBool(const string &name) const {
   map<string, bool>::const_iterator it(bool_properties_.find(name));
-  CHECK(it != bool_properties_.end());
+  CHECK(it != bool_properties_.end()) << "for bool property " << name;
   return it->second;
 }
 
 const string &KeyValueStore::GetString(const string &name) const {
   map<string, string>::const_iterator it(string_properties_.find(name));
-  CHECK(it != string_properties_.end());
+  CHECK(it != string_properties_.end()) << "for string property " << name;
   return it->second;
 }