ux: when attempting to install discontinued p2pool version

[plowsof]

i dont want to forget this small issue: if you try to install p2pool on an old version of the gui - usually the p2pool file is discontinued - the gui downloads this 'not found' file - and the hash check fails / tells the user to run the gui as administrator.
it should detect the p2pool file is not accessible and fail before checking hashes / show a more relevant error message