sslutil: check for missing certificate and key files (issue5598) Currently, sslutil._hostsettings() performs validation that web.cacerts exists. However, client certificates are passed in to the function and not all callers may validate them. This includes httpconnection.readauthforuri(), which loads the [auth] section. If a missing file is specified, the ssl module will raise a generic IOException. And, it doesn't even give us the courtesy of telling us which file is missing! Mercurial then prints a generic "abort: No such file or directory" (or similar) error, leaving users to scratch their head as to what file is missing. This commit introduces explicit validation of all paths passed as arguments to wrapsocket() and wrapserversocket(). Any missing file is alerted about explicitly. We should probably catch missing files earlier - as part of loading the [auth] section. However, I think the sslutil functions should check for file presence regardless of what callers do because that's the only way to be sure that missing files are always detected.