Add meaningful error messages

src/imapdomo.skel

        "are also available in .imapdomo/common.lua"
 }
 
+bug() {
+    #
+    # Die because of bug
+    #
+    local msg=$1
+    local self
+    self=$(basename "$0")
+    die "bug in $self (__MKIT_PROJ_VERSION__): $msg"
+}
+
 mkcmd() {
     #
     # Compose imapfilter command
      || dle "no handler for action: $Action.lua in $IMAPDOMO_HOME/handlers"
     cmd=$(mkcmd)
     debug -v cmd
-    bash -n <<<"$cmd" || die
+    bash -n <<<"$cmd" || bug "bad syntax of cmd: '$cmd'"
     if test -n "$CdTo";
     then
-        cd "$CdTo" || die
+        cd "$CdTo" || die "cannot chdir to: $CdTo"
     fi
-    mkdir -p "$HeaderDir" || die
+    mkdir -p "$HeaderDir" || die "cannot create header directory: $HeaderDir"
     eval "$cmd"
 }