We have a larger problem that passsing computed strings to the first
variable argument of many exception constructors is unsafe because that
first variable argument is interpreted not as a plain string, but format
string, and if it contains '%' boost::format will abort, since there are
no arguments to the format string.
In this particular instance '%' was used as part of an escape code in a
URL, which, when the download failed, caused Nix to abort displaying the
`FileTransferError`.