Additional error handling for rarer states.

Ensure std::map::at is not blindly applied without checking that the key
actually exists.

Fixes crashes where u2f client on PC/browser expects channel open, but
service has been restarted.

Streams.cpp

@@ -186,7 +186,10 @@ FILE* initHTML(FILE* fPtr, const string& title) {
 void closeHTML(FILE* fPtr) {
 	fprintf(fPtr, "\t</body>\n"
 	              "</html>");
-	fclose(fPtr);
+	int successCode = fclose(fPtr);
+
+	if (successCode != 0)
+		cerr << "File closing error: " << errno << endl;
 }
 
 #endif