Return more descriptive error codes on license generation failure

[ Merge from http://go/wvgerrit/14745 ]

License generation errors previously would result in code -2916 being returned
though the mediaDrm API. More descriptive error codes are now being returned
from -2850 to -2836

b/13976775

Change-Id: I613ad650ab0a072ce9d8029e2af52b72dc617236

libwvdrmengine/cdm/core/src/cdm_session.cpp

@@ -209,11 +209,12 @@ CdmResponseType CdmSession::GenerateKeyRequest(
     }
 
     app_parameters_ = app_parameters;
-    if (!license_parser_->PrepareKeyRequest(init_data, license_type,
-                                            app_parameters, session_id_,
-                                            key_request, server_url)) {
-      return KEY_REQUEST_ERROR_2;
-    }
+    CdmResponseType status = license_parser_->PrepareKeyRequest(
+                                 init_data, license_type,
+                                 app_parameters, session_id_,
+                                 key_request, server_url);
+
+    if (KEY_MESSAGE != status) return status;
 
     key_request_ = *key_request;
     if (is_offline_) {