Improve the error report of format

[Martin Edström]

Background: I am currently trying to find out where an error "Format
specifier doesn't match argument type" is coming from.  It's a bit of a
mystery, so I thought, hey, this could be a lot easier if the error
message includes the format string that it is trying to print!  That way
it would be a simple search across a codebase.

TL;DR: can format's error message include the input string?

Thanks,

Martin