Fix error message generation

Fix error message generation

The right-hand-side operand of % is supposed to be a tuple with the
same number of arguments as there are formatting codes in the
left-hand-side operand, or just any value if there is just one
formatting code; however, here it was a variable with a tuple value,
which didn't work. So wrap it in a tuple of length one, and all is
well again.

Signed-off-by: Karl Hasselström <kha@treskal.com>