basicfuncs+: Fix a think-o

When parsing a number, during any of the numeric template functions, and
the parse fails, display the error message correctly. Previously, the
same error message was displayed for both the first and the second
argument, and both messages said "first argument".

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #38 from algernon/h/num-parse-fixup

basicfuncs+: Fix a think-o