Make unavailable expl3 a fatal error

l3kernel is now in collection-latex and an update to expl3 triggers format rebuild, so it is safe to assume that expl3 is available.

base/ltexpl.dtx

@@ -140,15 +140,39 @@
           >0 %
           \expandafter\@firstofone
         \else
-          \message{Skipping expl3-dependent extensions}%
-%<2ekernel>          \expandafter\endinput
-%<latexrelease>          \expandafter\@gobble
+%    \end{macrocode}
+%
+% In \texttt{2ekernel} mode, an error is fatal and building the format
+% is aborted. Use \verb=\batchmode \read -1 to \tokenlist=, which errors
+% with
+% \verb=! Emergency stop. (cannot \read from terminal in nonstop modes)=
+% and aborts the \TeX{} run.  In \texttt{latexrelease} mode, raise an
+% error and do nothing.  Both ways, the error message shows the minimum
+% \textsf{expl3} engine requirements.
+%    \begin{macrocode}
+%<2ekernel>          \def~{ }\def\MessageBreak{^^J~~~~~~~~~~~~~~~}%
+%<2ekernel>          \errmessage{LaTeX Error:
+%<latexrelease>          \@latex@error{%
+            LaTeX requires the e-TeX primitives and additional\MessageBreak
+            functionality available in the engines:\MessageBreak
+              - pdfTeX v1.40\MessageBreak
+              - XeTeX v0.99992\MessageBreak
+              - LuaTeX v0.95\MessageBreak
+              - e-(u)pTeX mid-2012\MessageBreak
+            or later%
+%<latexrelease>          }\@ehd \expandafter\@gobble
+%<2ekernel>          }\batchmode \read -1 to \reserved@a
         \fi
       }
       {%
-        \message{Skipping expl3-dependent extensions}%
-%<2ekernel>        \endinput
-%<latexrelease>        \@gobble
+%<*2ekernel>
+        \errmessage{LaTeX requires expl3}%
+        \batchmode \read -1 to \reserved@a
+%</2ekernel>
+%<*latexrelease>
+        \@latex@error{LaTeX requires expl3}\@ehd
+        \@gobble
+%</latexrelease>
       }%
       {%
         \input expl3.ltx