Fix macro name for @sprintf errors

[simonster]

This is a bit pedantic, but before we'd get:

julia> @sprintf "%d"
ERROR: @printf: wrong number of arguments

[simonster]

Okay, updated to get the errors right in all conditions (I think!). I also fixed is_str_expr so that it actually triggers. (I suspect there have been some changes to AST structure since this was written.)

[ivarne]

See also #5847

[simonster]

Now includes a further fix: @sprintf notaformatstring now correctly prints "@sprintf: first argument must be a format string" instead of "@printf: first or second argument must be a format string".

[StefanKarpinski]

Man, I feel for GitHub users @printf and @sprintf...

[simonster]

Whoops, sorry guys.

[StefanKarpinski]

Testing now, but this all looks good to me.

[StefanKarpinski]

Thanks for fixing all that. We are not meticulously apt in our error messaging :-)

[simonster]

Thanks for reviewing!