more robust: avoid implicit setup (with default resolver);

--- a/lib/Tools/ghc	Wed Oct 17 22:36:51 2018 +0200
+++ b/lib/Tools/ghc	Wed Oct 17 22:41:59 2018 +0200
@@ -4,4 +4,9 @@
 #
 # DESCRIPTION: invoke Glasgow Haskell Compiler within the Isabelle environment
 
-isabelle_stack ghc -- "$@"
+if [ -d "$ISABELLE_STACK_ROOT" ]; then
+  isabelle_stack ghc -- "$@"
+else
+  exit 127
+fi
+

--- a/lib/Tools/ghci	Wed Oct 17 22:36:51 2018 +0200
+++ b/lib/Tools/ghci	Wed Oct 17 22:41:59 2018 +0200
@@ -4,4 +4,8 @@
 #
 # DESCRIPTION: invoke GHC interaction within the Isabelle environment
 
-isabelle_stack ghci "$@"
+if [ -d "$ISABELLE_STACK_ROOT" ]; then
+  isabelle_stack ghci "$@"
+else
+  exit 127
+fi