Added the exception handler
handle _ => exit 1. This will catch all errors and force an exit with error
code, causing Make to fail.

--- a/src/ZF/IMP/ROOT.ML	Mon Feb 27 17:44:43 1995 +0100
+++ b/src/ZF/IMP/ROOT.ML	Mon Feb 27 17:47:44 1995 +0100
@@ -17,4 +17,4 @@
 writeln"Root file for ZF/IMP";
 proof_timing := true;
 loadpath := [".","IMP"];
-time_use_thy "Equiv";
+time_use_thy "Equiv" handle _ => exit 1;