Define font_enable_not_found_error as a warning in pdfTeX too

In LuaTeX and XeTeX we define both \font_suppress_not_found_error: and
\font_enable_not_found_error:, which cannot be done with pdfTeX.  Only
the first was defined to give a warning.  It makes more sense to define
both.


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5253 de43f980-851b-0410-b2f7-c40aca1f87e0