Prevent '%' in URL from causing crashes

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`.

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