Repeat prompt for wallet path if invalid

simplewallet run without a wallet path argument should prompt again if
an invalid path was entered.

Validity here currently means the string isn't empty.

src/wallet/wallet2.h

@@ -225,6 +225,12 @@ namespace tools
     }
 
     static void wallet_exists(const std::string& file_path, bool& keys_file_exists, bool& wallet_file_exists);
+    /*!
+     * \brief  Check if wallet file path is valid format
+     * \param  file_path      Wallet file path
+     * \return                Whether path is valid format
+     */
+    static bool wallet_valid_path_format(const std::string& file_path);
 
     static bool parse_payment_id(const std::string& payment_id_str, crypto::hash& payment_id);