make holqbf executable print output when exceptions are raised

aids debugging, especially when running it as a cgi script

src/HolQbf/holqbf.sml

@@ -1,17 +1,22 @@
 open Opentheory Logging
+exception D
 fun main() = let
   exception E of term
   val t = ( raw_read_article TextIO.stdIn {
-    define_tyop = fn _ => raise Fail "define_tyop",
-    define_const = fn _ => raise Fail "define_const",
+    define_tyop = fn _ => (print "define_tyop"; raise D),
+    define_const = fn _ => (print "define_const"; raise D),
     axiom = fn _ => fn (_,c) => raise (E c),
     const_name = const_name_in_map,
-    tyop_name = tyop_name_in_map } ; raise Fail "no theorem" )
+    tyop_name = tyop_name_in_map } ; (print "no theorem"; raise D) )
   handle E t => t
   val _ = Feedback.emit_MESG := false
   val _ = raw_start_logging TextIO.stdOut
   val th = HolQbfLib.decide t
   val _ = export_thm th
   val _ = stop_logging()
 in () end
+handle HOL_ERR e => (print (format_ERR e); raise D)
+     | Io {name,function,cause=SysErr(s,_)} => (print ("IO error: name="^name^", function="^function^"SysErr="^s); raise D)
+     | Io {name,function,cause} => (print ("IO error: name="^name^", function="^function); raise D)
+     | e => (print "other error: "; PolyML.print e; raise D)
 val _ = PolyML.export("holqbf",main)