A call to Log::error() should be enough to indicate that it is an
error. We don't need to prefix the message with the string "Error: "
in some cases but not others. (If we do want such a prefix for all
errors, surely then we should add it in the actual Log::error()
function.)
Also, change some more Log::error() calls to Log::syserror() where
appropriate.
Much better than assuming that errno would be relevant at all
Log::error() calls (or alternatively, having to remember to append a
false parameter to the Log::error() call, which had not been done a
single time anyway.)
Call log::syserror() right after a system call has returned an
error. Don't call it otherwise.
