getdown launcher shouldn't trash getdown.txt when development build no longer available from build server

If a development build getdown chanel is no longer available (e.g. the build server has 'tidied up' a stale build) then various things may end up being retrieved from the URL that formerly returned a valid getdown.txt file.

The first time a corrupt getdown.txt is downloaded - Jalview is still launched, because the launcher uses the last valid getdown.txt file cached locally, but the next time a launch is attempted the 'digest file is invalid' warning is reported, and the launcher fails to offer any other option.

It would be great if the 'last good configuration' could always be launched in this situation, perhaps with a clear warning saying 'channel no longer available' !