Instruct users to run zcash-fetch-params if network params aren't available

Closes #1786.

src/init.cpp

@@ -621,6 +621,17 @@ static void ZC_LoadParams()
     boost::filesystem::path pk_path = ZC_GetParamsDir() / "sprout-proving.key";
     boost::filesystem::path vk_path = ZC_GetParamsDir() / "sprout-verifying.key";
 
+    if (!(boost::filesystem::exists(pk_path) && boost::filesystem::exists(vk_path))) {
+        uiInterface.ThreadSafeMessageBox(strprintf(
+            _("Cannot find the Zcash network parameters in the following directory:\n"
+              "%s\n"
+              "Please run 'zcash-fetch-params' and then restart."),
+                ZC_GetParamsDir()),
+            "", CClientUIInterface::MSG_ERROR);
+        StartShutdown();
+        return;
+    }
+
     pzcashParams = ZCJoinSplit::Unopened();
 
     LogPrintf("Loading verifying key from %s\n", vk_path.string().c_str());