Don't interpret strings as format strings

Ludo reported this error:

  unexpected Nix daemon error: boost::too_few_args: format-string refered to more arguments than were passed

coming from this line:

  printMsg(lvlError, run.program + ": " + string(err, 0, p));

The problem here is that the string ends up implicitly converted to a
Boost format() object, so % characters are treated specially.  I
always assumed (wrongly) that strings are converted to a format object
that outputs the string as-is.

Since this assumption appears in several places that may be hard to
grep for, I've added some C++ type hackery to ensures that the right
thing happens.  So you don't have to worry about % in statements like

  printMsg(lvlError, "foo: " + s);

or

  throw Error("foo: " + s);