Catch API Timeouts

[symptog]

Some tests may sometimes throw a timeout.

1) test Overpass.Parser with XML | Relation (OverpassParserTest)
     test/overpass_parser_test.exs:67
     ** (Protocol.UndefinedError) protocol String.Chars not implemented for %HTTPoison.Error{id: nil, reason: :timeout}
     stacktrace:
       (elixir) lib/string/chars.ex:3: String.Chars.impl_for!/1
       (elixir) lib/string/chars.ex:17: String.Chars.to_string/1
       (logger) lib/logger.ex:644: Logger.truncate/2
       (logger) lib/logger.ex:507: Logger.bare_log/3
       (overpass) lib/overpass/api.ex:56: Overpass.API.process_response/1
       test/overpass_parser_test.exs:72: (test)