Fixed a few error messages.

helper.zsh

@@ -19,12 +19,12 @@ TRAP_SIGNALS=(
 # Adds a function to a list to be called when a trap is triggered.
 function add-zsh-trap() {
   if (( $# < 2 )); then
-    print "usage: $0 type function"
+    print "usage: $0 type function" >&2
     return 1
   fi
 
   if [[ -z "$TRAP_SIGNALS[(r)$1]" ]]; then
-    print "$0: unknown signal: $1"
+    print "$0: unknown signal: $1" >&2
     return 1
   fi
 

init.zsh

@@ -3,7 +3,7 @@
 # Check for the minimum supported version.
 min_zsh_version='4.3.10'
 if ! autoload -Uz is-at-least || ! is-at-least "$min_zsh_version"; then
-  print "omz: too old shell version detected, minimum required: $min_zsh_version"
+  print "omz: old shell version detected, minimum required: $min_zsh_version" >&2
 fi
 unset min_zsh_version