better release lock file

cocktail

@@ -14,6 +14,7 @@ STORE=/tmp/exegetes
 
 die() {
   echo -e "$@" >&2
+  release_project
   exit 1
 }
 
@@ -223,7 +224,10 @@ lock_project() {
 
 release_project() {
   verbose "$FUNCNAME $PROJET released"
-  rm -f $LOCK_FILE
+  if test -e "$LOCKFILE";
+  then
+    rm -f $LOCK_FILE
+  fi
 }
 
 usage() {